INVESTIGACIÓN

Author(s):

V. Monteseguro Padron, J.A. Barreda Argüeso, J. Ruiz Fuertes, A.D. Rosa, H.L. Meyerheim, T. Irifune, F. Rodriguez

Source: Scientific Reports

Volume: 12   Páginas: 1217

Abstract: An advanced experimental and theoretical model to explain the correlation between the electronic and local structure of Eu2+ in two different environments within a same compound, EuS, is presented. EuX monochalcogenides (X: O, S, Se, Te) exhibit anomalies in all their properties around 14 GPa with a semiconductor to metal transition. Although it is known that these changes are related to the 4𝑓75𝑑0 → 4𝑓65𝑑1 electronic transition, no consistent model of the pressure-induced modifications of the electronic structure currently exists. We show, by optical and x-ray absorption spectroscopy, and by ab initio calculations up to 35 GPa, that the pressure evolution of the crystal field plays a major role in triggering the observed electronic transitions from semiconductor to the half-metal and finally to the metallic state.

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DOI: https://doi.org/10.1038/s41598-022-05321-9

Author(s): C. Martin Sanchez, A. Sánchez-Iglesias, P. Mulvaney, L. M. Liz-Marzán, F. Rodriguez

Source: Journal of Physical Chemistry C

Volume: 12 6(4)  Páginas: 1982-1990

Abstract: The effects of nonhydrostatic pressure on the morphology and stability of gold nanorods (AuNRs) and nanospheres (AuNSs) in 4:1 methanol–ethanol mixtures were studied by optical absorption spectroscopy and transmission electron microscopy at pressures of up to 23 and 30 GPa, respectively. Solvent solidification and associated nonhydrostatic stresses were found to have a negligible effect on the shape and size of AuNSs. On the contrary, while AuNRs maintained their initial morphology in the hydrostatic range, the uniaxial stress component induced under nonhydrostatic conditions had a shearing effect on the AuNRs, breaking them into smaller particles. Interestingly, colloidal stability was maintained in all cases, and the particles showed no sign of aggregation, despite the severe nonhydrostatic conditions to which both AuNR and AuNS colloids were subjected.

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DOI: https://doi.org/10.1021/acs.jpcc.1c10767

Author(s):

B. Zhong, A. Mateu-Roldán, M. Lopez Fanarraga, W. Han, D. Muñoz Guerra, J. Gonzalez, L. T. Weng, M. R. Ibarra, C. Marquina, K. L. Yeung

Source: Chemical Engineering Journal

Volume: 435  Páginas: 134466

Abstract: Iron nanoparticles encapsulated within graphene shells (Fe@C) were examined for cellular internalization, subcellular behavior, biocompatibility, and influence on cell viability and proliferation. Studies on human lung (adenocarcinoma human alveolar basal epithelial) and skin (epidermoid carcinoma) cells indicate Fe@C is less toxic and more biocompatible than the magnetite nanoparticles coated by an amorphous carbon (Fe₃O₄@C). Fe₃O₄@C exhibited more signs of degradation than Fe@C when exposed to murine macrophages (mouse monocyte-macrophages J774). Unlike Fe₃O₄@C, Fe@C has a high drug loading capacity (0.18 g/g) for ferulic acid, an active pharmaceutical ingredient found in the traditional Chinese herb Angelica sinensis and releases the drug at a constant dosing rate of 8.75 mg/g/day over 30 days. Ferulic acid released by Fe@C injected subcutaneously in diabetic BALB/c mice is effective in lowering the blood glucose level.

KeyWords Plus: Drug delivery, Diabetic mice, Carbon-coated magnetic nanoparticles, Graphene, Cytotoxicity

DOI: https://doi.org/10.1016/j.cej.2021.134466

Author(s):

R. Turnbull, J. Gonzalez-Platas, F. Rodríguez, A. Liang, C. Popescu, Z. He, D. Santamaría-Pérez, P. Rodríguez-Hernández, A. Muñoz, D. Errandonea

Source: Inorganic Chemistry

Volume: 61  Páginas: 3697−3707

Abstract: The understanding of the interplay between crystal structure and electronic structure in semiconductor materials is of great importance due to their potential technological applications. Pressure is an ideal external control parameter to tune the crystal structures of semiconductor materials in order to investigate their emergent piezo-electrical and optical properties. Accordingly, we investigate here the high-pressure behavior of the semiconducting antiferromagnetic material β-Cu₂V₂O₇, finding it undergoes a pressure-induced phase transition to γ-Cu₂V₂O₇ below 4000 atm. The pressure-induced structural and electronic evolutions are investigated by single-crystal X-ray diffraction, absorption spectroscopy and ab initio density functional theory calculations. β-Cu₂V₂O₇ has previously been suggested as a promising photocatalyst for water splitting. Now, these new results suggest that β-Cu₂V₂O₇ could also be of interest with regards to barocaloric effects, due to the low phase -transition pressure, in particular because it is a multiferroic material. Moreover, the phase transition involves an electronic band gap decrease of approximately 0.2 eV (from 1.93 to 1.75 eV) and a large structural volume collapse of approximately 7%.

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DOI: https://doi.org/10.1021/acs.inorgchem.1c03878

Author(s): V. Monteseguro, J. Ruiz-Fuertes, J. A. Barreda-Argüeso, H. L. Meyerheim, A. D. Rosa, F. Rodríguez

Source: Physical Review Letters

Volume: 128  Páginas: 099701

Abstract:

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DOI: https://doi.org/10.1103/PhysRevLett.128.099701

Author(s): A. Liang, F. Rodríguez, P. Rodríguez-Hernandez, A. Muñoz, R. Turnbull, and D. Errandonea

Source: Physical Review B

Volume: 105 (11)  Páginas: 115204

Abstract: High-pressure optical-absorption measurements performed on polycrystalline Co(IO3)2 samples were used to characterize the influence of pressure on the electronic d–d transitions associated with Co2+ and the fundamental band gap of Co(IO3)2. The results shed light on the electron-lattice coupling and show that Co(IO3)2 exhibits an unusual behavior because the compression of Co–O bond distances is not coupled to pressure-induced changes induced in the unit-cell volume. Experimental results on the internal d–d transitions of Co2+ have been explained based on changes in the constituent CoO6 octahedral units using the semiempirical Tanabe-Sugano diagram. Our findings support that the high-spin ground state (T14) is very stable in Co(IO3)2. We have also determined the band-gap energy of Co(IO3)2 and its pressure dependence which is highly nonlinear. According to density-functional theory band-structure calculations, this nonlinearity occurs because the bottom of the conduction band is dominated by I-5p orbitals and the top of the valence band by Co-3d and O-2p orbitals, and because the Co–O and I–O bond lengths exhibit different pressure dependences.

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DOI: 10.1103/PhysRevB.105.115204

Author(s): M. T. Candela, F. Aguado, A. Diego-Rucabado, J. A. González, and R. Valiente

Source: Journal of Alloys and Compounds

Volume: 921  Páginas: 166043

Abstract: The structural and spectroscopic properties of (Eu_1-xYb_x)₂O₃ nanocrystals with cubic (C-type) and monoclinic (B-type) crystalline structures have been studied. NCs have been synthetized by the sol-gel Pechini method and characterized at room temperature by X-ray diffraction, transmission electron microscopy, diffuse reflectance, Raman spectroscopy and photoluminescence techniques. NIR emission from Yb³⁺ ions has been observed in both C- and B-type NCs upon excitation of Eu³⁺ ions at 532 nm, where Yb³⁺ ions do not absorb photons. This fact reveals that an efficient non-resonant energy transfer process from Eu³⁺ to Yb³⁺ takes place, allowing to obtain simultaneous visible and NIR emissions under visible excitation. The decay curves of the ⁵D₀ → ⁷F₂ Eu³⁺ emission of C-type NCs corroborate this phenomenon since the Eu³⁺ lifetime has been found to decrease as the Yb³⁺ content increases. Finally, we discuss the use of the Eu³⁺ luminescence as a structural probe to distinguish between different RE₂O₃ polymorphs.

KeyWords Plus: Rare Earth sesquioxides, Polymorphism, (Eu_1-xYb_x)₂O₃, Nanocrystals, Photoluminescence, Non-resonant Energy transfer

DOI: https://doi.org/10.1016/j.jallcom.2022.166043

Author(s): M. T. Candela, F. Aguado, A. Diego-Rucabado, J. A. González, and R. Valiente

Source: Journal of Alloys and Compounds

Volume: 921  Páginas: 166043

Abstract: The structural and spectroscopic properties of (Eu_1-xYb_x)₂O₃ nanocrystals with cubic (C-type) and monoclinic (B-type) crystalline structures have been studied. NCs have been synthetized by the sol-gel Pechini method and characterized at room temperature by X-ray diffraction, transmission electron microscopy, diffuse reflectance, Raman spectroscopy and photoluminescence techniques. NIR emission from Yb³⁺ ions has been observed in both C- and B-type NCs upon excitation of Eu³⁺ ions at 532 nm, where Yb³⁺ ions do not absorb photons. This fact reveals that an efficient non-resonant energy transfer process from Eu³⁺ to Yb³⁺ takes place, allowing to obtain simultaneous visible and NIR emissions under visible excitation. The decay curves of the ⁵D₀ → ⁷F₂ Eu³⁺ emission of C-type NCs corroborate this phenomenon since the Eu³⁺ lifetime has been found to decrease as the Yb³⁺ content increases. Finally, we discuss the use of the Eu³⁺ luminescence as a structural probe to distinguish between different RE₂O₃ polymorphs.

KeyWords Plus: Rare Earth sesquioxides, Polymorphism, (Eu_1-xYb_x)₂O₃, Nanocrystals, Photoluminescence, Non-resonant Energy transfer

DOI: https://doi.org/10.1016/j.jallcom.2022.166043

Author(s): R. Martín-Rodríguez, F. Aguado, M. D. Alba, R. Valiente, E. Pavón, A. C. Perdigón

Source: Journal of Alloys and Compounds

Volume: 921  Páginas: 166086

Abstract: High charge mica Na₄Al₄Si₄Mg₆O₂₀F₄, Mica-4, is a promising candidate as a filling material to immobilize high-level radioactive waste in deep geological repositories due to its extraordinary adsorption capacity. In contrast to traditional clay materials, the structural composition of this mica, with a high content of aluminum in the tetrahedral sheet, enhances its chemical reactivity, favoring the formation of new crystalline phases under mild hydrothermal conditions, and thus providing a definitive isolation of the radionuclides in the engineered barrier. Moreover, this synthetic clay has some features that allow its use as an optical sensor by doping with luminescent rare earth cations such as Eu³⁺. In this paper we discuss the local structure of the nanoclay Mica-4 using Eu³⁺ as a local probe to track the physical and chemical modifications under hydrothermal conditions. For that purpose, a set of hydrothermal experiments has been carried out heating Mica-4 and an aqueous Eu(NO₃)₃ solution in a stainless steel reactor at different temperatures and times. Optical properties of the as-treated samples were characterized by spectroscopic measurements. The fine peak structure of emission and the relative intensity of different Eu³⁺ transitions as well as the luminescence lifetime have been correlated with the structure and composition of this nanoclay, and the interaction mechanisms between the lanthanide ions and the clay mineral at different temperatures and times. Special attention has been paid to understanding the role of the aluminum content, which may act as either an aggregating or dispersing agent, in the optical features and reactivity of the system.

KeyWords Plus: Deep Geological Repository, Adsorption, Luminescence, Optical sensor, Radionuclide, High-charge mica

DOI: https://doi.org/10.1016/j.jallcom.2022.166086

Author(s): E. Jara, R. Valiente, J. González, J. I. Espeso, N. Khaidukov, and F. Rodríguez

Source: Journal of Materials Chemistry C

Volume: 10 (16)  Páginas: 6380-6391

Abstract: We present a spectroscopic study of the doubly Mn⁴⁺ and Cr³⁺-doped Sr₄Al₁₄O₂₅, synthesised via solid state reaction, as a function of pressure and temperature to check its potential as a red emitting phosphor. The strong crystal field acting on Cr³⁺ yields a ruby-like narrow R-line emission that is used as an intrinsic temperature sensor and, in comparison to ruby, provides a fair estimate of the bulk modulus K = 172 GPa for Sr₄Al₁₄O₂₅. The photoluminescence (PL) and associated excitation spectra of Sr₄Al₁₄O₂₅:Mn⁴⁺,Cr³⁺ unravel a strong-field 3d³ configuration with R₁ and R₂ emission lines largely separated by 10.7 meV, which characterizes the non-centrosymmetric distorted Al4 site in the Sr₄Al₁₄O₂₅ structure. Eighteen Raman modes have been detected at ambient pressure. They correspond to the stretching modes of the Al–O bonds and show frequency shifts with pressure consistent with Grüneisen parameters (γ = 0.65), similar to those measured for other aluminates. The variations of PL intensity and lifetime with temperature exhibit a similar concomitant behaviour indicating an efficient pumping to the ⁴T₂ level and hence down to the ²E emitting level in both Mn⁴⁺ and Cr³⁺ ions at low and moderate temperatures. Multiphonon non-radiative processes yield a quenching temperature of 400 K for Mn⁴⁺ with an effective activation energy of 0.57 eV. Unexpectedly, this energy decreases with pressure enhancing the non-radiative processes and thus PL intensity reduction. A thoroughly coordinate configurational energy diagram is presented for explaining the main spectroscopic features and excited-state dynamics.

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DOI: https://doi.org/10.1039/D2TC00485B

Author(s): E. Navarro-Palomares, L. García-Hevia, J. Galán-Vidal, A. Gandarillas, F. García-Reija, A. Sánchez-Iglesias, L. M. Liz- Marzan, R. Valiente and M. L. Fanarraga

Source: International J. Nanomedicine

Volume:   Páginas: Aceptado

Abstract: A great challenge in nanomedicine, and more specifically in theranostics, is to improve the specificity, selectivity, and targeting of nanomaterials towards target tissues or cells. The topical use of nanomedicines as adjuvants to systemic chemotherapy can significantly improve the survival of patients affected by localized carcinomas, reducing the side effects of traditional drugs and preventing local recurrences.

KeyWords Plus: unctionalized nanomaterial; globotriaosylceramide; nanoparticle targeting; natural ligand; squamous carcinoma.

DOI: 10.2147/IJN.S381628

Author(s): C. Martín-Sánchez, A. Sánchez-Iglesias, J.A. Barreda-Argüeso, A Polian, L. M. Liz-Marzán, F. Rodríguez

Source: ACS Nano

Volume:   Páginas: Aceptado

Abstract: The mechanical properties and stability of metal nanoparticle colloids under high-pressure conditions are investigated by means of optical extinction spectroscopy and small-angle X-ray scattering (SAXS), for colloidal dispersions of gold nanorods and gold nanospheres. SAXS allows us to follow in situ the structural evolution of the nanoparticles induced by pressure, regarding both nanoparticle size and shape (form factor) and their aggregation through the interparticle correlation function S(q) (structure factor). The observed behavior changes under hydrostatic and nonhydrostatic conditions are discussed in terms of liquid solidification processes yielding nanoparticle aggregation. We show that pressure-induced diffusion and aggregation of gold nanorods take place after solidification of the solvent. The effect of nanoparticle shape on the aggregation process is additionally discussed.

KeyWords Plus: gold nanoparticles, high-pressure, small-angle X-ray scattering, aggregation, pressure-induced diffusion

DOI: https://doi.org/10.1021/acsnano.2c10643

Author(s): J.A. Barreda-Argüeso, F. Rodríguez

Source: Physical Review B

Volume: 103   Páginas: 085115

Abstract: This work investigates the Ni-F distance dependence of the crystal-field (CF) transitions of Ni2+ in KNiF3 by high-pressure spectroscopy. All peaks shift to higher energy with pressure according to trends foreseen by the Tanabe-Sugano diagram. At ambient conditions, we obtain Racah and CF splitting parameters of B=0.118eV, 10Dq=0.908eV; C/B=4.4 (10Dq/B=7.7). B and 10Dq vary with pressure as ∂B∂P=–0.11meVGPa–1 and ∂10Dq∂P=24meVGPa–1. Similar to KCoF3, the slight decrease of B with pressure reflects the strong ionic character of the Ni-F bond and its high stability against compression. We have correlated the measured pressure dependence of 10Dq with the Ni-F bond distance, showing that it follows a potential law as 10Dq=CR–n with an exponent n=6.6±0.5, thus providing experimental data for checking the suitability of theoretical models aiming to explain the slight deviations of observed R dependencies of 10Dq from the CF theory (n=5). We have applied the experimental 10Dq(R) relationship to determine the real Ni-F bond distances in fluoroperovskites ABF3:Ni2+ from the spectroscopically measured 10Dq as an alternative method for determining bond distances, RNi−F, in impurity systems. We show that the so-obtained RNi−F deviates from the bond distance of the host site, RB−F, proportionally to the difference RB−F−R0, with R0 being the sum of ionic radii RF−+RNi2+. The behavior is compared to that found for Mn2+ along the fluoroperovskite series ABF3:Mn2+. Finally, weak UV peaks observed below the charge-transfer band gap (Eg≤10eV) in the absorption spectrum, the assignment of which still remains controversial, have been assigned to single and double excitation transitions. The assignment was unveiled on the basis of their energy and pressure shift.

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DOI: https://doi.org/10.1103/PhysRevB.103.085115

Author(s): J. Pellicer-Porres, A. Segura, Ch. Ferrer-Roca, J. A. Gonzalez, V. Muñoz-San jose

Source: Journal of Alloys and Compounds

Volume: 852   Páginas: 156365

Abstract: In this work we study the local structure of layered alloys by means of X-ray Absorption. We complement our research with a Raman study. The available alloys have compositions close to the host binaries. The dependence of XANES (X-ray Near Edge Structure) features with synchrotron polarization clearly establishes the substitutional character of the alloying cations. Interstitial atoms, if present, remain under the detection limit of the technique. EXAFS (Extended X-ray Absorption Fine Structure) demonstrates the existence of bonds in the alloys. The cation-anion bond-lengths in the alloys remain similar to that of the pure compounds. There is no significant increase of the static disorder. The basic features of Raman spectra are those of the pure compounds, with the Raman peaks shifting continuously as the impurity cation is introduced. New features associated to defects are discussed.

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Layered semiconductor, X-ray absorption, XANES, EXAFS, Raman, Alloy

DOI: https://doi.org/10.1103/PhysRevB.103.085115

Author(s): L. Valdivia, L. García-Hevia, M. Bañobre-López, J. Gallo, R. Valiente

Source: Pharmaceutics

Volume: 13   Páginas: 93

Abstract: Solid lipid particles (SLPs) can sustainably encapsulate and release therapeutic agents over long periods, modifying their biodistribution, toxicity, and side effects. To date, no studies have been reported using SLPs loaded with doxorubicin chemotherapy for the treatment of metastatic cancer. This study characterizes the effect of doxorubicin-loaded carnauba wax particles in the treatment of lung metastatic malignant melanoma in vivo. Compared with the free drug, intravenously administrated doxorubicin-loaded SLPs significantly reduce the number of pulmonary metastatic foci in mice. In vitro kinetic studies show two distinctive drug release profiles. A first chemotherapy burst-release wave occurs during the first 5 h, which accounts for approximately 30% of the entrapped drug rapidly providing therapeutic concentrations. The second wave occurs after the arrival of the particles to the final destination in the lung. This release is sustained for long periods (>40 days), providing constant levels of chemotherapy in situ that trigger the inhibition of metastatic growth. Our findings suggest that the use of chemotherapy with loaded SLPs could substantially improve the effectiveness of the drug locally, reducing side effects while improving overall survival.

KeyWords Plus: cancer; doxorubicin; drug delivery; melanoma; nanomedicine.

DOI: 10.3390/pharmaceutics13010093

Author(s): A Diego-Rucabado, M.T. Candela, F. Aguado, J. González, E. Gómez, R. Valiente, I. Cano, R. Martín-Rodríguez

Source: Reaction Chemistry & Engineering

Volume: 6   Páginas: 2376-2390

Abstract: A series of undoped and transition-metal (TM)-doped TiO₂ nanocrystals (NCs) were synthesized and calcined at different temperatures, and fully characterized. Such NCs were employed as catalysts for the photodegradation of methylene blue, which enabled us to study the influence of both NC size and anatase/brookite/rutile phase ratio on the photocatalytic activity, as well as the effect of different TM dopants, namely Mn and Co. Then, the NCs were used as active additives for the fabrication of a new photocatalytic system composed of an enamel incorporating these NCs supported onto a stainless-steel sheet. NCs both in powder form and incorporated in enamels deposited on steel were characterized by transmission electron microscopy, X-ray diffraction, and reflectance and Raman spectroscopy. We demonstrate how the calcination of TiO₂ NCs induces both a growth in the anatase ratio and formation of the rutile form, which leads to a photocatalytic activity increase. Similarly, doping with Mn and Co gives rise to an enhancement of the catalytic performance attributed to a displacement of the energy bandgap. The obtained material combines the resistance of steel and the photocatalytic activity of TiO₂ deposited on enamel, which also operates as a corrosion protection layer for the former. The resulting smart photocatalytic surface presents many applications such as a self-cleaning coating and potential use for NOx photodegradation.

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DOI: https://doi.org/10.1039/D1RE00293G

Author(s):

A. Andrada-Chacón, Á. Morales-García, M. A. Salvadó, P. Pertierra, R. Franco, G. Garbarino, M. Taravillo, J. A. Barreda-Argüeso, J. González, Valentín García Baonza, J. Manuel Recio, Javier Sánchez-Benítez

Source: Inorg. Chem.

Volume: 60, 3   Páginas: 1746-1754

Abstract: The quest for new transition metal dichalcogenides (TMDs) with outstanding electronic properties operating under ambient conditions draws us to investigate the 1T-HfSe₂ polytype under hydrostatic pressure. Diamond anvil cell (DAC) devices coupled to in situ synchrotron X-ray, Raman, and optical (VIS–NIR) absorption experiments along with density functional theory (DFT)-based calculations prove that (i) bulk 1T-HfSe₂ exhibits strong structural and vibrational anisotropies, being the interlayer direction especially sensitive to pressure changes, (ii) the indirect gap of 1T-HfSe₂ tends to vanish by a −0.1 eV/GPa pressure rate, slightly faster than MoS₂ or WS₂, (iii) the onset of the metallic behavior appears at P_met ∼10 GPa, which is to date the lowest pressure among common TMDs, and finally, (iv) the electronic transition is explained by the bulk modulus B₀–P_met correlation, along with the pressure coefficient of the band gap, in terms of the electronic overlap between chalcogenide p-type and metal d-type orbitals. Overall, our findings identify 1T-HfSe₂ as a new efficient TMD material with potential multipurpose technological applications.

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DOI: https://doi.org/10.1021/acs.inorgchem.0c03223

Author(s):

D. Gonzalez-Alonso, J. González, H. Gavilan, J. Fock, L. Zeng, K. Witte, P. Bender, L. Fernandez-Barquin, C. Johansson

Source: RSC Advances

Volume: 11   Páginas: 390

Abstract: The attractive electronic and magnetic properties together with their biocompatibility make iron-oxide nanoparticles appear as functional materials. In Fe-oxide nanoparticle (IONP) ensembles, it is crucial to enhance their performance thanks to controlled size, shape, and stoichiometry ensembles. In light of this, we conduct a comprehensive investigation in an ensemble of ca. 28 nm cuboid-shaped IONPs in which all the analyses concur with the coexistence of magnetite/maghemite phases in their cores. Here, we are disclosing the Verwey transition by temperature dependent (4–210 K) Raman spectroscopy.

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DOI: https://doi.org/10.1039/D0RA09226F

Author(s): M.T. Candela, F. Aguado, J.A. Gonzalez, R. Valiente

Source: Journal of Alloys and Compounds

Volume: 859   Páginas: 157899

Abstract: The attractive electronic and magnetic properties together with their biocompatibility make iron-oxide nanoparticles appear as functional materials. In Fe-oxide nanoparticle (IONP) ensembles, it is crucial to enhance their performance thanks to controlled size, shape, and stoichiometry ensembles. In light of this, we conduct a comprehensive investigation in an ensemble of ca. 28 nm cuboid-shaped IONPs in which all the analyses concur with the coexistence of magnetite/maghemite phases in their cores. Here, we are disclosing the Verwey transition by temperature dependent (4–210 K) Raman spectroscopy.

KeyWords Plus: Phosphors, High-pressure, Phase transitions, Luminescence, Inelastic light scattering

DOI: https://doi.org/10.1016/j.jallcom.2020.157899

Author(s):

A.O. Fumega, D. Wong, C. Schulz, F. Rodríguez, S. Blanco-Canosa

Source: Journal of Physics: Condensed Matter

Volume: 33(49)   Páginas: 495603

Abstract: We investigate the electronic structure of Cs₂CuCl₄, a material discussed in the framework of a frustrated quantum antiferromagnet, by means of resonant inelastic x-ray scattering (RIXS) and density functional theory (DFT). From the non-dispersive highly localized dd excitations, we resolve the crystal field splitting of the Cu²⁺ ions in a strongly distorted tetrahedral coordination. This allows us to model the RIXS spectrum within the crystal field theory (CFT), assign the dd orbital excitations and retrieve experimentally the values of the crystal field splitting parameters D_q, D_s and D_τ. The electronic structure obtained ab-initio agrees with the RIXS spectrum and modelled by CFT, highlighting the potential of combined spectroscopic, cluster and DFT calculations to determine the electronic ground state of complex materials.

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DOI: 10.1088/1361-648X/ac2648

Author(s): C. Renero, A. Herrero, D. Jimenez, M. Martínez-Flórez, R. Valiente, M. Mychinko, S. Bals, L. M. Liz-Marzán

Source: The Journal of Physical Chemistry C

Volume: 125   Páginas: 19887-19896

Abstract: The development of optical nanothermometers operating in the near-infrared (NIR) is of high relevance toward temperature measurements in biological systems. We propose herein the use of Nd³⁺-doped lanthanum oxychloride nanocrystals as an efficient system with intense photoluminescence under NIR irradiation in the first biological transparency window and emission in the second biological window with excellent emission stability over time under 808 nm excitation, regardless of Nd³⁺ concentration, which can be considered as a particular strength of our system. Additionally, surface passivation through overgrowth of an inert LaOCl shell around optically active LaOCl/Nd³⁺ cores was found to further enhance the photoluminescence intensity and also the lifetime of the 1066 nm, ⁴F_3/2 to ⁴I_11/2 transition, without affecting its (ratiometric) sensitivity toward temperature changes. As required for biological applications, we show that the obtained (initially hydrophobic) nanocrystals can be readily transferred into aqueous solvents with high, long-term stability, through either ligand exchange or encapsulation with an amphiphilic polymer.

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DOI: https://doi.org/10.1021/acs.jpcc.1c05828

Author(s): E. Navarro, L. García-Hevia, E. Padín-González, M. Bañobre-López, J.C. Villegas, R. Valiente, M.L. Fanarraga

Source: Cancers

Volume: 13   Páginas: 4920 (1-14)

Abstract: Head and Neck Cancer (HNC) is the seventh most common cancer worldwide with a 5-year survival from diagnosis of 50%. Currently, HNC is diagnosed by a physical examination followed by an histological biopsy, with surgery being the primary treatment. Here, we propose the use of targeted nanotechnology in support of existing diagnostic and therapeutic tools to prevent recurrences of tumors with poorly defined or surgically inaccessible margins. We have designed an innocuous ligand-protein, based on the receptor-binding domain of the Shiga toxin (ShTxB), that specifically drives nanoparticles to HNC cells bearing the globotriaosylceramide receptor on their surfaces. Microscopy images show how, upon binding to the receptor, the ShTxB-coated nanoparticles cause the clustering of the globotriaosylceramide receptors, the protrusion of filopodia, and rippling of the membrane, ultimately allowing the penetration of the ShTxB nanoparticles directly into the cell cytoplasm, thus triggering a biomimetic cellular response indistinguishable from that triggered by the full-length Shiga toxin. This functionalization strategy is a clear example of how some toxin fragments can be used as natural biosensors for the detection of some localized cancers and to target nanomedicines to HNC lesions.

KeyWords Plus: oral cancer; nanotechnology; toxin; recombinant ligand-protein

DOI: https://doi.org/10.3390/cancers13194920

Author(s): F. Aguado, R. Martín-Rodríguez, C. Pesquera, R. Valiente, A. C. Perdigón

Source: Nanomaterials

Volume: 11   Páginas: 3167, 1-13

Abstract: A versatile, functional nanomaterial for the removal of ionic and non-ionic pollutants is presented in this work. For that purpose, the high charge mica Na-4-Mica was exchanged with the cationic surfactant (C₁₆H₃₃NH(CH₃)₂)⁺. The intercalation of the tertiary amine in the swellable nano-clay provides the optimal hydrophilic/hydrophobic nature in the bidimensional galleries of the nanomaterial responsible for the dual functionality. The organo-mica, made by functionalization with C₁₆H₃₃NH₃⁺, was also synthesized for comparison purposes. Both samples were characterized by X-ray diffraction techniques and transmission electron microscopy. Then, the samples were exposed to a saturated atmosphere of cyclohexylamine for two days, and the adsorption capacity was evaluated by thermogravimetric measurements. Eu³⁺ cations served as a proof of concept for the adsorption of ionic pollutants in an aqueous solution. Optical measurements were used to identify the adsorption mechanism of Eu³⁺ cations, since Eu³⁺ emissions, including the relative intensity of different f-f transitions and the luminescence lifetime, can be used as an ideal spectroscopic probe to characterize the local environment. Finally, the stability of the amphiphilic hybrid nanomaterial after the adsorption was also tested.

KeyWords Plus: Eu3+ luminescence; adsorption; calorimetry; decontamination; high charge mica; ionic pollutants; non-ionic pollutants.

DOI: 10.3390/nano11123167.

Author(s):

S. Gallego-Parra, R. Vilaplana, O. Gomis, E. Lora Da Silva, A. Otero-De-La-Roza, P. Rodríguez-Hernández, A. Muñoz, J. A. Gonzalez, J. A. Sans, V. P. Cuenca-Gotor, J. Ibáñez, C. Popescu, F. J. Manjón

Source: Physical Chemistry Chemical Physics

Volume: 12   Páginas: 6841-6862

Abstract: We report a joint experimental and theoretical study of the low-pressure phase of α′-Ga₂S₃ under compression. Theoretical ab initio calculations have been compared to X-ray diffraction and Raman scattering measurements under high pressure carried out up to 17.5 and 16.1 GPa, respectively. In addition, we report Raman scattering measurements of α′-Ga₂S₃ at high temperature that have allowed us to study its anharmonic properties. To understand better the compression of this compound, we have evaluated the topological properties of the electron density, the electron localization function, and the electronic properties as a function of pressure. As a result, we shed light on the role of the Ga–S bonds, the van der Waals interactions inside the channels of the crystalline structure, and the single and double lone electron pairs of the sulphur atoms in the anisotropic compression of α′-Ga₂S₃. We found that the structural channels are responsible for the anisotropic properties of α′-Ga₂S₃ and the A′(6) phonon, known as the breathing mode and associated with these channels, exhibits the highest anharmonic behaviour. Finally, we report calculations of the electronic band structure of α′-Ga₂S₃ at different pressures and find a nonlinear pressure behaviour of the direct band gap and a pressure-induced direct-to-indirect band gap crossover that is similar to the behaviour previously reported in other ordered-vacancy compounds, including β-Ga₂Se₃. The importance of the single and, more specially, the double lone electron pairs of sulphur in the pressure dependence of the topmost valence band of α′-Ga₂S₃ is stressed.

KeyWords Plus:

DOI: https://doi.org/10.1039/D0CP06417C

Author(s): N. Iturrioz, R. Martin-Rodriguez, C. Renero-Lecuna, F. Aguado, L. Gonzalez-Legarreta J. A. Gonzalez, et al.

Source: Applied Surface Science

Volume: 537   Páginas: 147870

Abstract: Laponite is a nanoplatform that has been successfully used as a new biomaterial for drug delivery, tissue engineering and bioimaging at the nanoscale. In general, a deep knowledge of the mechanism interaction of the nanomaterial with biological components in a physiological environment is highly desirable for properly characterizing its therapeutic efficacy and toxicology. Up to know, the use of fluorescent dyes labelling both, the nanomaterial and cell components, has been a requirement to characterize the cell uptake and to visualize the entrance of the nanomaterial into the cytosol and the cell nucleus. The used of fluorophores usually perturb the physiological medium and can interfere in the nanomaterial cell interaction. A new Raman imaging methodology to track the uptake and internalization of Laponite nanoparticles into J774 macrophages line cells is presented in this work. The combination of Raman spectroscopy and confocal microscopy provides direct information about the localization of the nanoparticle into the cell, through its unique vibrational fingerprint without labelling or adding dyes, and taking advantage of the fact that Laponite and biological molecules bands can be clearly differentiated.

KeyWords Plus: Laponite, Nanomaterial, Nanocarrier, Raman imaging technique, Nanoclay

DOI: https://doi.org/10.1016/j.apsusc.2020.147870

Author(s): V. Monteseguro, V. Venkatramu, U.R. Rodríguez-Mendoza, V. Lavín

Source: Dalton Transactions

Volume: 50(27)   Páginas: 9512-9518

Abstract: The green, red, near-infrared and near-infrared-to-visible upconverted luminescence properties of Er³⁺/Yb³⁺ codoped Y₃Ga₅O₁₂ nanocrystalline powders have been studied using laser spectroscopy. A diffuse reflectance and luminescence spectra confirm that Er³⁺ and Yb³⁺ ions occupy the Y³⁺ sites of the single-phase cubic nano-garnet. Very bright green and red luminescence of the Er³⁺ ions are detected by the naked eyes, even for a laser power as low as 15 mW, when the Yb³⁺ ions are excited at 970 nm. The red upconverted emission is more intense than that under direct excitation of the Er³⁺ ions. The power dependence and the dynamics of the near-infrared-to-green and near-infrared-to-red upconverted emissions show the existence of different two-photon energy transfer upconversion processes. The results here presented indicate that Er³⁺/Yb³⁺ codoped Y₃Ga₅O₁₂ can be a good candidate as an optical nanoheater and nanothermometer in biomedicine applications in the first biological window.

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DOI: https://doi.org/10.1039/D1DT00976A

Author(s): V. Monteseguro, V. Venkatramu, U.R. Rodríguez-Mendoza, V. Lavín

Source: Dalton Transactions

Volume: 50(16)   Páginas: 5493-5505

Abstract: The structural, vibrational and electronic properties of the compressed β-Sb₂O₃ polymorph, a.k.a. mineral valentinite, have been investigated in a joint experimental and theoretical study up to 23 GPa. The compressibility of the lattice parameters, unit-cell volume and polyhedral unit volume as well as the behaviour of its Raman- and IR-active modes under compression have been interpreted on the basis of ab initio theoretical simulations. Valentinite shows an unusual compressibility up to 15 GPa with four different pressure ranges, whose critical pressures are 2, 4, and 10 GPa. The pressure dependence of the main structural units, the lack of soft phonons, and the electronic density charge topology address the changes at those critical pressures to isostructural phase transitions of degree higher than 2. In particular, the transitions at 2 and 4 GPa can be ascribed to the changes in the interaction between the stereochemically-active lone electron pairs of Sb atoms under compression. The changes observed above 10 GPa, characterized by a general softening of several Raman- and IR-active modes, point to a structural instability prior to the 1st-order transition occurring above 15 GPa. Above this pressure, a tentative new high-pressure phase (s.g. Pcc2) has been assigned by single-crystal and powder X-ray diffraction measurements.

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DOI: https://doi.org/10.1039/D1DT00976A

Author(s): E. Jara, R. Valiente, M. Bettinelli, F. Rodríguez

Source: The Journal of Physical Chemistry C

Volume: 125   Páginas: 27118–27129

Abstract: We present a spectroscopic study of Mn-doped Mg2TiO4 as a function of pressure and temperature to check its viability as a red-emitting phosphor. The synthesis following a solid-state reaction route yields not only the formation of Mn4+ but also small traces of Mn3+. Although we show that Mn4+ photoluminescence is not appreciably affected by the presence of Mn3+, its local structure at the substituted Ti4+ host site causes a reduction of the Mn4+ pumping efficiency yielding a drastic quantum-yield reduction at room temperature. By combining Raman and time-resolved emission and excitation spectroscopies, we propose a model for explaining the puzzling nonradiative and inefficient pumping processes attained in this material. In addition, we unveil a structural phase transition above 14 GPa that worsens their photoluminescence capabilities. The decrease of emission intensity and lifetime with increasing temperature following different thermally activated de-excitation pathways is mostly related to relatively small activation energies and the electric−dipole transition mechanism associated with coupling to odd-parity vibrational modes. A thorough model based on the configurational energy level diagram to the A1g normal mode fairly accounts for the observed excitation and emissionthe quantum yieldof this material.

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DOI: 10.1021/acs.jpcc.1c08006

Author(s):

C. Martín-Sánchez, A. Sánchez-Iglesias, J.A. Barreda-Argüeso, A Polian, J.P. Itié, J. Pérez, P. Mulvaney, L. M. Liz-Marzán, F. Rodríguez

Source: ACS Nano

Volume: 15   Páginas: 19128–19137

Abstract: The density and compressibility of nanoscale gold (both nanospheres and nanorods) and microscale gold (bulk) were simultaneously studied by X-ray diffraction with synchrotron radiation up to 30 GPa. Colloidal stability (aggregation state and nanoparticle shape and size) in both hydrostatic and nonhydrostatic regions was monitored by small-angle X-ray scattering. We demonstrate that nonhydrostatic effects due to solvent solidification had a negligible influence on the stability of the nanoparticles. Conversely, nonhydrostatic effects produced axial stresses on the nanoparticle up to a factor 10× higher than those on the bulk metal. Working under hydrostatic conditions (liquid solution), we determined the equation of state of individual nanoparticles. From the values of the lattice parameter and bulk modulus, we found that gold nanoparticles are slightly denser (0.3%) and stiffer (2%) than bulk gold: V₀ = 67.65(3) ų, K₀ = 170(3)GPa, at zero pressure.

KeyWords Plus: gold nanoparticles, hydrostatic pressure, nonhydrostatic effects, X-ray diffraction, small-angle X-ray scattering, specific volume at nanoscale, bulk modulus

DOI: 10.1021/acs.jpcc.1c08006

Author(s): E. Haussühl, L. Bayarjargal, J. Ruiz-Fuertes

Source: Journal of Applied Physics

Volume: 129(14)   Páginas: Journal of Applied Physics

Abstract:

The elastic properties of 𝛾-LiAlO2 were reinvestigated with the aid of resonant ultrasound spectroscopy (RUS) at ambient conditions. A strong discrepancy of the elastic coefficients derived by RUS can be found from the experimental results from the literature, where 𝑐12 and 𝑐13 deviate from our results by up to 60% (46 GPa). In contrast to the experimental 𝑐𝑖𝑗 from the literature, we can recognize a good agreement between the elastic coefficients derived from RUS and the values using density functional theory. The dielectric permittivity was measured on large plane-parallel plates, and the piezoelectric stress coefficient 𝑒123=0.14Cm−2 was derived from RUS measurements at ambient conditions. The heat capacity between 4 and 398 K has been obtained by microcalorimetry using a relaxation calorimeter. The Debye temperature was derived from heat capacity measurements (ΘCp=676 K) and RUS measurements (Θelastic=688 K).

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DOI: 10.1021/acs.jpcc.1c08006

Author(s): R. Chuliá-Jordán, D. Santamaria-Perez, J. Ruiz Fuertes, A. Otero-De-La-Roza, C. Popescu

Source: Minerals

Volume: 11(6)   Páginas: 607

Abstract:

The structure of the naturally occurring, iron-richmineral Ca1.08(6)Mg0.24(2)Fe0.64(4)Mn0.04(1)(CO3)2 ankerite was studied in a joint experimental and computational study. Synchrotron X-ray powder diffraction measurements up to 20 GPa were complemented by density functional theory calculations. The rhombohedral ankerite structure is stable under compression up to 12 GPa. A third-order Birch–Murnaghan equation of state yields V0 = 328.2(3) Å3, bulk modulus B0 = 89(4) GPa, and its first-pressure derivative B’0 = 5.3(8)—values which are in good agreement with those obtained in our calculations for an ideal CaFe(CO3)2 ankerite composition. At 12 GPa, the iron-rich ankerite structure undergoes a reversible phase transition that could be a consequence of increasingly non-hydrostatic conditions above 10 GPa. The high-pressure phase could not be characterized. DFT calculations were used to explore the relative stability of several potential high-pressure phases (dolomite-II-, dolomite- III- and dolomite-V-type structures), and suggest that the dolomite-V phase is the thermodynamically stable phase above 5 GPa. A novel high-pressure polymorph more stable than the dolomite-III-type phase for ideal CaFe(CO3)2 ankerite was also proposed. This high-pressure phase consists of Fe and Ca atoms in sevenfold and ninefold coordination, respectively, while carbonate groups remain in a trigonal planar configuration. This phase could be a candidate structure for dense carbonates in other compositional systems.

KeyWords Plus: iron-rich ankerite; carbonate mineral; high pressure; phase transition; compressibility

DOI: https://doi.org/10.3390/min11060607

Author(s): R. Chuliá-Jordán, D. Santamaria-Perez, J. Ruiz Fuertes, A. Otero-De-La-Roza, C. Popescu

Source: ACS Earth and Space Chemistry

Volume: 5  Páginas: 1130-1139

Abstract:

New single-crystal X-ray diffraction experiments and density functional theory (DFT) calculations reveal that the crystal chemistry of the CaO–BaO–CO₂ system is more complex than previously thought. We characterized the BaCa(CO₃)₂ alstonite structure at ambient conditions, which differs from the recently reported crystal structure of this mineral in the stacking of the carbonate groups. This structural change entails the existence of different cation coordination environments. The structural behavior of alstonite at high pressures was studied using synchrotron powder X-ray diffraction data and ab initio calculations up to 19 and 50 GPa, respectively. According to the experiments, above 9 GPa, the alstonite structure distorts into a monoclinic C2 phase derived from the initial trigonal structure. This is consistent with the appearance of imaginary frequencies and geometry relaxation in DFT calculations. Moreover, calculations predict a second phase transition at 24 GPa, which would cause the increase in the coordination number of Ba atoms from 10 to 11 and 12. We determined the equation of state of alstonite (V₀ = 1608(2) ų, B₀ = 60(3) GPa, B′₀ = 4.4(8) from experimental data) and analyzed the evolution of the polyhedral units under compression. The crystal chemistry of alstonite was compared to that of other carbonates and the relative stability of all known BaCa(CO₃)₂ polymorphs was investigated.

KeyWords Plus:

alstonite, BaCa(CO₃)₂, crystal structure, carbonate, sphase transition, high pressure, synchrotron X-ray diffraction, DFT calculations

DOI: https://doi.org/10.3390/min11060607

Author(s):

J. A. Sans, F. J. Manjón, A. L. De Jesús Pereira, J. Ruiz Fuertes, C. Popescu, A. Muñoz, P. Rodríguez-Hernández, J. Pellicer-Porres, V. P. Cuenca-Gotor, J. Contreras-García, J. Ibañez, V. Monteseguro Padron

Source: Dalton Transaction

Volume: 50 (6)  Páginas: 5493-5505

Abstract:

The structural, vibrational and electronic properties of the compressed β-Sb₂O₃ polymorph, a.k.a. mineral valentinite, have been investigated in a joint experimental and theoretical study up to 23 GPa. The compressibility of the lattice parameters, unit-cell volume and polyhedral unit volume as well as the behaviour of its Raman- and IR-active modes under compression have been interpreted on the basis of ab initio theoretical simulations. Valentinite shows an unusual compressibility up to 15 GPa with four different pressure ranges, whose critical pressures are 2, 4, and 10 GPa. The pressure dependence of the main structural units, the lack of soft phonons, and the electronic density charge topology address the changes at those critical pressures to isostructural phase transitions of degree higher than 2. In particular, the transitions at 2 and 4 GPa can be ascribed to the changes in the interaction between the stereochemically-active lone electron pairs of Sb atoms under compression. The changes observed above 10 GPa, characterized by a general softening of several Raman- and IR-active modes, point to a structural instability prior to the 1st-order transition occurring above 15 GPa. Above this pressure, a tentative new high-pressure phase (s.g. Pcc2) has been assigned by single-crystal and powder X-ray diffraction measurements.

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DOI:

DOIhttps://doi.org/10.1039/D1DT00268F

Author(s): P. Gonzalez Izquierdo, O. Fabelo Rosa, L.a Cañadillas-Delgado, G. Beobide, O. Vallcorba, J. Salgado-Beceiro, M. Sánchez-Andújar, C. Martín, J. Ruiz Fuertes, J. Eduardo García, M. Teresa Fernández-Díaz, M. De Pedro Del Valle

Source: Journal of Materials Chemistry C

Volume: 9 (13)  Páginas: 4453-4465

Abstract: Quinuclidinium salts and their derivatives are now in the focus of materials science as building units of multifunctional materials. Their properties can be easily switchable, allowing their use in a wide range of physical applications. One type of these kinds of materials, the homochiral hybrid halometallate ferroelectric compounds, is not well understood. In this work, (R)-(−)-3-quinuclidinol hydrochloride was used in the synthesis of ((R)-(−)-3-hydroxyquinuclidium)[FeCl₄]. The use of this enantiomeric cation forces crystallographic non-centrosymmetry, which was confirmed by polarimetry and circular dichroism spectroscopy. We studied the physical properties of this compound at different temperatures by single crystal, synchrotron and neutron powder X-ray diffraction, which showed a rich series of structural and magnetic phase transitions. From synchrotron powder X-ray diffraction data, a plastic phase was observed above 370 K (phase I). Between 370 K and ca. 310 K, an intermediate polar phase was detected, solved in a non-centrosymmetric polar space group (C2) (phase II). Below ca. 310 K, the compound crystallizes in the triclinic P1 non-centrosymmetric space group (phase III) which is maintained down to 4 K, followed by phase IV, which shows tridimensional magnetic ordering. The temperature evolution of the neutron diffraction data shows the appearance of new reflections below 4 K. These reflections can be indexed to a commensurate propagation vector k = (0, 0, ½). The magnetic structure below T_N was solved in the P_s1 Shubnikov space group, which gives rise to an antiferromagnetic structure, compatible with the magnetometry measurements. Near room temperature, the crystal phase transition is associated with a dielectric change. In particular, the phase transition between phase III (S.G.:P1) and phase II (S.G.:C2) involves an increase of symmetry between two non-centrosymmetric space groups. Therefore, it allows, by symmetry, the emergence of ferroelectric and ferroelastic ordering. Piezoresponse force microscopy (PFM) imaging measurements provided evidence for polarization switching and a local ferroelectric behavior of phase III at room temperature. Additionally, the obtained butterfly curve and hysteresis loop by PFM exhibits a low coercive voltage of ∼10 V. This value is remarkable, since it approaches those obtained for materials with application in ferroelectric random access memories (FeRAMs).

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DOI https://doi.org/10.1039/D0TC05800A

Author(s): M. T. Candela, F. Aguado, J. González-Lavín, J. A. González, and R. Valiente

Source: Journal of Alloys and Compounds

Volume: 859  Páginas: 157899

Abstract: The high-pressure behavior of cubic Tb₂O₃ phase (C-type) has been investigated by in situ photoluminescence and Raman spectroscopy up to 18 GPa and 22 GPa, respectively. The luminescence spectrum of Tb₂O₃ at ambient conditions shows its characteristic sharp-line emission bands with the most intense one corresponding to the ⁵D₄ – ⁷F₅ transition, responsible for the green emission. Excitation and emission spectra measured at low temperature and at different emission and excitation wavelengths suggest that photoluminescence is governed by the Tb³⁺ in the non-centrosymmetric site (C₂). A phase transition between 7.2 and 8.6 GPa is observed by Raman and photoluminescence experiments and is associated with a cubic-to-monoclinic (B-type) structural transformation. From 14 GPa the predominant modes in the Raman spectrum correspond to those from the trigonal phase (A-type), indicating that the pressure-induced structural phase transition sequence of Tb₂O₃ is C → B → A. Upon pressure release, the starting cubic phase is not recovered, but the monoclinic one.

KeyWords Plus: Phosphors, High-pressure, Phase transitions, Luminescence, Inelastic light scattering

DOI: https://doi.org/10.1016/j.jallcom.2020.157899

Author(s): A. Barreda-Argüeso, and F. Rodríguez

Source: Physical Review B

Volume: 103(8)  Páginas: 085115

Abstract: This work investigates the Ni-F distance dependence of the crystal-field (CF) transitions of Ni2+ in KNiF3 by high-pressure spectroscopy. All peaks shift to higher energy with pressure according to trends foreseen by the Tanabe-Sugano diagram. At ambient conditions, we obtain Racah and CF splitting parameters of B=0.118eV, 10Dq=0.908eV; C/B=4.4 (10Dq/B=7.7). B and 10Dq vary with pressure as ∂B∂P=–0.11meVGPa–1 and ∂10Dq∂P=24meVGPa–1. Similar to KCoF3, the slight decrease of B with pressure reflects the strong ionic character of the Ni-F bond and its high stability against compression. We have correlated the measured pressure dependence of 10Dq with the Ni-F bond distance, showing that it follows a potential law as 10Dq=CR–n with an exponent n=6.6±0.5, thus providing experimental data for checking the suitability of theoretical models aiming to explain the slight deviations of observed R dependencies of 10Dq from the CF theory (n=5). We have applied the experimental 10Dq(R) relationship to determine the real Ni-F bond distances in fluoroperovskites ABF3:Ni2+ from the spectroscopically measured 10Dq as an alternative method for determining bond distances, RNi−F, in impurity systems. We show that the so-obtained RNi−F deviates from the bond distance of the host site, RB−F, proportionally to the difference RB−F−R0, with R0 being the sum of ionic radii RF−+RNi2+.

KeyWords Plus: High-pressure, Phase transitions

DOI:10.1103/PhysRevB.103.085115

Author(s): E. Padín-González, E. Navarro-Palomares, L. Valdivia, N. Iturrioz-Rodríguez, M. A. Correa, R. Valiente, M. L. Fanarraga

Source: Nanomedicine, Nanotechnology, Biology, and Medicine

Abstract: Here we propose a one-step strategy to endow nanomaterials with a custom-designed bio-identity. This study designs a universal ‘nanomaterial binding domain’ that can be genetically attached to any protein ensuring precise and spontaneous protein orientation. We demonstrate how, despite the simplicity of the method, the bioconjugation achieved: (i) is highly efficient, even in the presence of competing proteins, (ii) is stable at extreme physiological conditions (pH ranges 5.2-9.0; NaCl concentrations 0-1 M); (iii) prevents unwanted protein biofouling days after incubation in biologically-relevant conditions; and finally, (iv) avoids nanoparticle interaction with promiscuous unspecific receptors. In summary, this protein biocoating technique, applicable to a wide array of nano-designs, integrates material science and molecular biology procedures to create hybrid nanodevices with well-defined surfaces and predictable biological behaviors, opening a chapter in precision nanodiagnostics, nanosensing or nanotherapeutic applications.

KeyWords Plus: Biofouling; Biotechnology; Chimera protein; Electrostatic interaction; Nano–bio interface.

DOI: 10.1016/j.nano.2020.102268

Author(s): J. A. Barreda-Argüeso, J. González, R. Valiente, T. Irifune and F. Rodríguez

Source: High Pressure Research

Volume: 40   Páginas: 117-118

Abstract: We investigated the optical properties (absorption, luminescence and Raman spectra) of nanopolycrystalline diamond (NPD) aiming at exploring its capabilities as a pressure sensor and as a pressure-cell anvil for combined X-ray/neutron and optical studies. Notably, we analysed the Raman peak shift and broadening with pressure using a Moissanite Anvil Cell (MAC). The results are compared with those obtained in a DAC, where Raman signals from NPD chips and diamond anvils strongly overlap. Its pressure behaviour in the hydrostatic and non-hydrostatic regimes were investigated. We showed that the nanopolycrystalline structure induces remarkable differences in the peak shift and broadening between NPD and natural type IIa single-crystal diamond, making NPD suitable as pressure gauge for pressure determination and testing hydrostaticity of pressure transmitting medium.

KeyWords Plus: Raman spectroscopy Luminescence Pressure gauge Pressure sensitivity Nanopolycrystalline diamond

DOI: 10.1080/08957959.2019.1702173

Author(s): C. Martín-Sánchez, A. Sánchez-Iglesias, P. Mulvaney, L. M. Liz-Marzán, and F. Rodríguez

Source: The Journal of Physical Chemistry C

Volume: 124 (16)   Páginas: 8978-8983

Abstract: The localized surface plasmon resonance (LSPR) of gold nanospheres dispersed in methanol–ethanol 4:1 was measured as a function of pressure up to 60 GPa. The LSPR exhibits an intense red-shift with pressure in the range of 0–10 GPa, followed by a slower blue-shift at higher pressures. This is because an increase in the solvent refractive index with pressure leads to a red-shift of the LSPR peak wavelength while an increase in the electron density of the gold nanospheres with pressure leads to a blue-shift. Solvent solidification at 10 GPa and associated nonhydrostatic effects have a negligible influence on the LSPR shifts in the case of nanospheres. Here we show that both the LSPR shifts and changes in the nanospheres absorption coefficient can be explained on the basis of Gans’ model, and this enables the solvent refractive index and the density of the solvent to be determined across the hydrostatic pressure range from 0 to 60 GPa. Interestingly, plasmonic sensing shows no evidence of crystallization or glass phase transitions in MeOH–EtOH 4:1 within the explored pressure range.

KeyWords Plus: Metal nanoparticles, Gold, Optical properties, Surface plasmon resonance, Solvents

DOI: https://doi.org/10.1021/acs.jpcc.0c01419

Author(s): E. Navarro-Palomares, P. González-Saiz, C. Renero-Lecuna, R. Martín-Rodríguez, F. Aguado, D. González-Alonso, L. Fernández-Barquín, J. González, M. Bañobre-López, M. L. Fanarraga, R. Valiente

Source: Nanoscale

Volume: 12   Páginas: 378876164-6175

Abstract: In vivo imaging and therapy represent one of the most promising areas in nanomedicine. Particularly, the identification and localization of nanomaterials within cells and tissues are key issues to understand their interaction with biological components, namely their cell internalization route, intracellular destination, therapeutic activity and possible cytotoxicity. Here, we show the development of multifunctional nanoparticles (NPs) by providing luminescent functionality to zinc and iron oxide NPs. We describe simple synthesis methods based on modified Stöber procedures to incorporate fluorescent molecules on the surface of oxide NPs. These procedures involve the successful coating of NPs with size-controlled amorphous silica (SiO₂) shells incorporating standard chromophores like fluorescein, rhodamine B or rhodamine B isothiocyanate. Specifically, spherical Fe₃O₄ NPs with an average size of 10 nm and commercial ZnO NPs (ca. 130 nm), both coated with an amorphous SiO₂ shell of ca. 15 and 24 nm thickness, respectively, are presented. The magnetic nanoparticles, with a major presence of magnetite, show negligible coercitivity. Hence, interactions (dipolar) are very weak and the cores are in the superparamagnetic regime. Spectroscopic measurements confirm the presence of fluorescent molecules within the SiO₂ shell, making these hybrid NPs suitable for bioimaging. Thus, our coating procedures improve NP dispersibility in physiological media and allow the identification and localization of intracellular ZnO and Fe₃O₄ NPs using confocal microscopy imaging preserving the fluorescence of the NP. We demonstrate how both Fe₃O₄ and ZnO NPs coated with luminescent SiO₂ are internalized and accumulated in the cell cytoplasm after 24 hours. Besides, the SiO₂ shell provides a platform for further functionalization that enables the design of targeted therapeutic strategies. Finally, we studied the degradation of the shell in different physiological environments, pointing out that the SiO₂ coating is stable enough to reach the target cells maintaining its original structure. Degradation took place only 24 hours after exposure to different media.

KeyWords Plus:

DOI: https://doi.org/10.1039/C9NR08743E

Author(s): C. McMonagle, P. Comar, G. S. Nichol, D. R. Allan, J. A. Barreda-Argūeso, J. A. Gonzalez, F. Rodriguez, R. Valiente, G. Turner, E. K. Brechin and St. A. Moggach

Source: Chemical Science

Volume: 11   Páginas: 3788793-8799

Abstract: In situ single-crystal diffraction and spectroscopic techniques have been used to study a previously unreported Cu-framework bis[1-(4-pyridyl)butane-1,3-dione]copper(II) (CuPyr-I). CuPyr-I was found to exhibit high-pressure and low-temperature phase transitions, piezochromism, negative linear compressibility, and a pressure induced Jahn–Teller switch, where the switching pressure was hydrostatic media dependent.

KeyWords Plus:

DOI: https://doi.org/10.1039/D0SC03229H

Author(s): R. Valiente and F. Rodriguez

Source: JACS Applied Materials & Interfaces

Volume: 12   Páginas: 37807-37810

Abstract: In recent years, two-dimensional perovskites have received considerable attention for their potential applications for optoelectronics. Contrary to previous publications, we demonstrate that (CH₃NH₃)₂CuCl₄ hybrid organic–inorganic layered perovskite does not show any room-temperature photoluminescence (PL) under UV excitation. This statement can be extended to other perovskites with general formula AMX₃ or A₂MX₄, based on M: Cu²⁺ and X: Cl^– or Br^–. These materials, the object of increasing interest because of their efficient light absorption in a wide UV–vis–NIR range ideal for solar cells and optoelectronics, lack PL at room temperature, in contrast to recent findings reporting PL properties in this and other similar Cu²⁺-related materials.

KeyWords Plus: Absorption, Crystal structure, Solar cells, Optoelectronics, Perovskites

DOI: https://doi.org/10.1021/acsami.0c11480

Author(s): C. Martín-Sánchez, S. Seibt, J. A. Barreda-Argüeso and F. Rodríguez

Source: Journal of Physics: Conference Series

Volume: 1609(1)   Páginas: 012009 (7p)

Abstract:

The sensing capabilities of gold nanorods under high-pressure conditions were investigated in methanol-ethanol mixtures (up to 13 GPa) and in water (up to 9 GPa) through their optical extinction. The longitudinal SPR band of AuNR exhibits a redshift with pressure which is the result of two main competing effects: compression of the conduction electrons which increases the bulk plasma frequency (blueshift) and increase in the solvent density (redshift). The variation in de SPR peak wavelength allows us to estimate the bulk modulus of the gold nanoparticles with a precision of 10 % and to obtain analytical functions providing the pressure dependence of the refractive index of water in three phases: liquid, ice VI and ice VII. Furthermore, the SPR band shows abrupt jumps at the liquid to ice phase VI and ice phase VII transitions, which are in accordance with the first-order character of these transitions.

KeyWords Plus:

DOI: 10.1088/1742-6596/1609/1/012009

Author(s): V. Gutiérrez-Cano, R. Valiente, J.A. González, and F. Rodriguez

Source: Journal of Physics: Conference Series

Volume: 1609(1)   Páginas: 12004 (9p)

Abstract:

The optical properties of a sparsely investigated material, LaGdO3 doped with Er3+, are explored regarding its suitability as nanothermometer. Besides its excellent capabilities for dielectric applications, when doping with Er3+, this material provides a highly efficient upconversion photoluminescence (PL) for high temperature thermometry at high pressure due to its structural stability. LaGdO3 belongs to the perovskite-type ABO3 compounds with a B-type monoclinic C2/m space group (a = 14.43 Å; b = 3.69 Å; c = 9.00 Å; and β = 100.70º) at ambient conditions. It undergoes a structural phase transition to a hexagonal 𝑃3#𝑚1 phase at 3 GPa yielding a notable PL enhancement, thus enabling it as a potential high-pressure hightemperature nanothermometer.

KeyWords Plus:

DOI: doi:10.1088/1742-6596/1609/1/012004

Author(s): E. Jara, J. González, F. Aguado, R. Valiente and F. Rodríguez

Source: Journal of Physics: Conference Series

Volume: 1609(1)   Páginas: 12005 (7p)

Abstract: ^{The effects of pressure in electronic and vibrational properties of the double perovskite CaCu3Ti4O12 have been investigated in the 0-25 GPa range by optical absorption and Raman spectroscopy. Besides a full structural characterization, we aim at unveiling whether the ambient Im3 crystal structure is stable under high pressure conditions and how its giant dielectric permitivity and electronic gap varies with pressure. Results show that there is evidence of neither structural phase transition nor metallization in CaCu3Ti4O12 in the explored pressure range. We have observed the eight Raman active modes associated with its Im3 crystal phase and obtained their corresponding frequency and pressure shift. Moreover, the direct electronic band gap (2.20 eV), which is mainly associated with the oxygen-to-copper charge transfer states, increases slightly with pressure at a rate of 13 meV GPa?1 from 0 to 10 GPa. Above this pressure is almost constant (Eg = 2.3 eV). The results highlight the high stability of the compound in its Im3 phase against compression.}

KeyWords Plus:DOI:

DOI: 10.1088/1742-6596/1609/1/012005

Author(s): I. Fernández, R. Valiente, F. Ortiz, C. J. Renedo, A. Ortiz

Source: Nanomaterials

Volume: 10   Páginas: 692 (18p)

Abstract: ^{Over the last few decades the insulating performance of transformer oils has been broadly studied under the point of view of nanotechnology, which tries to improve the insulating and heat dissipation performance of transformer oils by suspending nanoparticles. Many authors have analyzed the thermal and dielectric behavior of vegetable oil based-nanofluids, however, very few works have studied the evolution of these liquids during thermal aging and their stability. In this paper has been evaluated the performance of aged vegetable oil based-nanofluids, which have been subjected to accelerated thermal aging at 150 °C. Nanoparticles of TiO₂ and ZnO have been dispersed in a commercial natural ester. Breakdown voltage, resistivity, dissipation factor and acidity of nanofluid samples have been measured according to standard methods, as well as stability. Moreover, it has been analyzed the degradation of Kraft paper through the degree of polymerization (DP). The results have showed that although nanoparticles improve breakdown voltage, they increase the ageing of insulation liquids and dielectric paper.}

KeyWords Plus: Energy and Catalysis

 DOI: https://doi.org/10.3390/nano10040692

Author(s): A. Diego-Rucabado, M. T. Candela, F. Aguado, J. González, F. Rodríguez, R. Valiente, R. Martín-Rodríguez, I. Cano

Source: Nanomaterials

Volume: 10   Páginas: 1574 (20p)

Abstract: ^{Pr3+-doped Y₂O₃ nanocrystals (NCs) have been obtained via five wet-chemistry synthesis methods which were optimized in order to achieve superior optical properties. To this end, a systematic study on the influence of different reaction parameters was performed for each procedure. Specifically, precursor concentration, reaction temperature, calcination temperature, and time, among others, were analyzed. The synthesized Y₂O₃: Pr3+ NCs were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and reflectance and Raman spectroscopy. In addition, the optical properties of such NCs were investigated by excitation, emission, and luminescence decay measurements. Concretely, emission from the 1D₂ level was detected in all samples, while emission from 3P_J was absent. Finally, the effect of the synthesis methods and the reaction conditions on the luminescence decay has been discussed, and a comparative study of the different methods using the fluorescence lifetime of so-obtained Y₂O₃: Pr3+ NCs as a figure of merit has been carried out}

KeyWords Plus: Síntesis and Characterization of Nanoparticles with Luminescence Properties

DOI: https://doi.org/10.3390/nano10081574

Author(s): Yiwei Sun, Paolo Passaretti, Ignacio Hernandez, Jesus Gonzalez, Wei Liu, Fernando Rodriguez, David J. Dunstan, Pola Goldberg O ppenheimer6 & Colin J. Humphreys

Source: Scientific reports

Volume: 10(1)   Páginas: 1-8

Abstract: Graphene oxide, integrated with the filamentous bacteriophage M13, forms a 3D large-scale multifunctional porous structure by self-assembly, with considerable potential for applications. We performed Raman spectroscopy under pressure on this porous composite to understand its fundamental mechanics. The results show that at low applied pressure, the 𝑠𝑝2 bonds of graphene oxide stiffen very little with increasing pressure, suggesting a complicated behaviour of water intercalated between the graphene layers. The key message of this paper is that water in a confined space can have a significant impact on the nanostructure that hosts it. We introduced carbon nanotubes during the self-assembly of graphene oxide and M13, and a similar porous macro-structure was observed. However, in the presence of carbon nanotubes, pressure is transmitted to the 𝑠𝑝2 bonds of graphene oxide straightforwardly as in graphite. The electrical conductivity of the composite containing carbon nanotubes is improved by about 30 times at a bias voltage of 10 V. This observation suggests that the porous structure has potential in applications where good electrical conductivity is desired, such as sensors and batteries.

KeyWords Plus:

DOI: https://doi.org/10.1038/s41598-020-72372-1

Author(s): M. T. Candela, E. Jara, F. Aguado, R. Valiente, and F. Rodríguez

Source: Inorganic Chemistry

Volume: 124    Páginas: 22692-22703

Abstract: An experimental correlation study between the low-symmetry 3d orbital splitting pattern, Δ_e and Δ_t, determined by optical spectroscopy, and the local distortion ρ, determined by X-ray diffraction, for different Cu²⁺ and Mn³⁺ fluorides and chlorides is presented. Single crystals of different dimensionalities were explored, some of them studied under high-pressure conditions. The collection of structural and spectroscopic data provides structural correlations relating ρ and Δ_e and Δ_t in Cu²⁺ and Mn³⁺ systems, showing that Δ_e (and Δ_t) scales with ρ. Such correlations can be used to estimate local distortions of Cu²⁺ (or Mn³⁺) introduced as impurities in different chloride and fluoride host lattices from spectroscopic data. The results can be interpreted in the framework of the Jahn–Teller theory and provide support for the proposed structural scenario. The influence of the crystal anisotropy in the local structure is analyzed as well and compared with transition metal ion systems having E⊗e Jahn–Teller (Cu²⁺, Mn³⁺) and singly orbital (Ni²⁺, Mn²⁺, Fe³⁺) ground states in octahedral symmetry.

KeyWords Plus: Group theory, Crystals, Anions, Transition metals, Ions

DOI: https://doi.org/10.1021/acs.jpcc.0c07243

Author(s): E. González-Lavado, L. Valdivia, A. Garcia-Castaño, F. González, C. Pesquera, R. Valiente, and M.L. Fanarraga

Source: Oncotarget

Volume: 10    Páginas: 2022-2029

Abstract: Isolation of high-level radioactive waste (HLW) in deep geological repositories (DGR) through a multibarrier concept is the most accepted approach to ensure long-term safety. Clay minerals are one of the most promising materials to be used as engineered barriers. In particular, high charge micas, as components of the engineered barrier, show superselectivity for some radioactive isotopes and a large adsorption capacity, which is almost twice that of the other low charge aluminosilicates. In addition, high charge micas are optimum candidates for decontamination of nuclear waste through two different mechanisms; namely an ion exchange reaction and a nonreversible mechanism involving the formation of new stable crystalline phases under hydrothermal conditions. In this work, we report a new in situ optical sensor based on the incorporation of Eu3+ in these high charge micas for tracking the long-term physical-chemical behavior of HLW contaminants in DRG under mild hydrothermal conditions. The incorporation of Eu3+ into the interlayer space of the mica originates a well resolved green and red luminescence, from both the 5D1 and 5D0 excited states, respectively. The formation of new crystalline phases under hydrothermal conditions involves important changes in the Eu3+ emission spectra and lifetime. The most interesting features of Eu3+ luminescence to be used as an optical sensor are (1) the presence or absence of the Eu3+ green emission from the 5D1 excited state, (2) the energy shift of the 5D0 → 7F0 transition, (3) the crystal-field splitting of the 7F1 Eu3+ level, and (4) the observed luminescence lifetimes, which are directly related to the interaction mechanisms between the lanthanide ions and the silicate network

KeyWords Plus: combined therapy; drug delivery; microtubule dynamics; nanocarrier; nanomaterial.

DOI: 10.18632/oncotarget.26770

Author(s): R. Martín-Rodríguez, F. Aguado, M.D. Alba, R. Valiente, and A.C. Perdigón

Source: ACS applied materials & interfaces

Volume: 11    Páginas: 7559-7565

Abstract:

Isolation of high-level radioactive waste (HLW) in deep geological repositories (DGR) through a multibarrier concept is the most accepted approach to ensure long-term safety. Clay minerals are one of the most promising materials to be used as engineered barriers. In particular, high charge micas, as components of the engineered barrier, show superselectivity for some radioactive isotopes and a large adsorption capacity, which is almost twice that of the other low charge aluminosilicates. In addition, high charge micas are optimum candidates for decontamination of nuclear waste through two different mechanisms; namely an ion exchange reaction and a nonreversible mechanism involving the formation of new stable crystalline phases under hydrothermal conditions. In this work, we report a new in situ optical sensor based on the incorporation of Eu3+ in these high charge micas for tracking the long-term physical-chemical behavior of HLW contaminants in DRG under mild hydrothermal conditions. The incorporation of Eu3+ into the interlayer space of the mica originates a well resolved green and red luminescence, from both the 5D1 and 5D0 excited states, respectively. The formation of new crystalline phases under hydrothermal conditions involves important changes in the Eu3+ emission spectra and lifetime. The most interesting features of Eu3+ luminescence to be used as an optical sensor are (1) the presence or absence of the Eu3+ green emission from the 5D1 excited state, (2) the energy shift of the 5D0 → 7F0 transition, (3) the crystal-field splitting of the 7F1 Eu3+ level, and (4) the observed luminescence lifetimes, which are directly related to the interaction mechanisms between the lanthanide ions and the silicate network

KeyWords Plus:

DOI: http://dx.doi.org/10.1021/acsami.8b20030

Author(s): E. Jara, J. A. Barreda-Argüeso, J. A. González, F. Rodríguez and R. Valiente

Source: Physical Review B

Volume: 13    Páginas: 134106 (9 p)

Abstract:

By using optical absorption and Raman spectroscopy, we have investigated the effects of pressure in the
structural, electronic, and vibrational properties of Cs2CuCl4 in the 0–20 GPa range. Our study has provided a complete characterization of the electronic and vibrational structures of Cs2CuCl4 in the low-pressure Pnma phase as a function of cell volume and the Cu-Cl bond length, RCu-Cl.We have focused on the electronic structure in the Pnma phase, which is mainly related to the tetrahedral CuCl2− 4 units distorted by the Jahn-Teller effect.
The strong piezochromism of Cs2CuCl4 is caused by the opposite shift of the charge-transfer band gap and the Cu2+ d-d crystal-field transitions with pressure. The high-pressure structure of Cs2CuCl4 above 4.9 GPa yields structural transformations that we ascribe to a change of coordination around Cu2+. The high-pressure phase appears largely amorphized, therefore any structural information from x-ray diffraction is ruled out. Here we use electronic and vibrational probes to get inside the structural information.

KeyWords Plus:

DOI: 10.1103/PhysRevB.99.134106

Author(s): J. A. Barreda-Argüeso, L. Nataf, F. Aguado, I. Hernández, J. González, A. Otero-de-la-Roza, V. Luaña, Y. Jia, C. Jin, Bongjae Kim, K. Kim, B. I. Min, W. Heribert, A. P. Jephcoat, and F. Rodríguez

Source: Scientific Reports

Volume: 9    Páginas: 5448

Abstract: The interplay between spin states and metallization in compressed CoCl₂ is investigated by combining diffraction, resistivity and spectroscopy techniques under high-pressure conditions and ab-initio calculations. A pressure-induced metallization along with a Co²⁺ high-spin (S = 3/2) to low-spin (S = 1/2) crossover transition is observed at high pressure near 70 GPa. This metallization process, which is associated with the p–d charge-transfer band gap closure, maintains the localization of 3d electrons around Co²⁺, demonstrating that metallization and localized Co²⁺ -3d low-spin magnetism can coexist prior to the full 3d-electron delocalization (Mott-Hubbard d-d breakdown) at pressures greater than 180 GPa.

KeyWords Plus:

DOI: https://doi.org/10.1038/s41598-019-41337-4

Author(s): Y. W. Sun, W. Liu, I. Hernandez, J. Gonzalez, F. Rodriguez, D. J. Dunstan, and C. J. Humphreys

Source: Physical Review Letters

Volume: 123    Páginas: 135501

Abstract: This work addresses a fundamental question: To what extent is graphene graphite? In particular does 2D
graphene have many of the same 3D mechanical properties as graphite, such as the bulk modulus and
elastic constant c33? We have obtained, for the first time, unambiguous Raman spectra from unsupported
monolayer graphene under pressure. We have used these data to quantify the out-of-plane stiffness of
monolayer graphene, which is hard to define due to its 2D nature. Our data indicate a first physically
meaningful out-of-plane stiffness of monolayer graphene, and find it to be consistent with that of graphite.
We also report a shift rate of the in-plane phonon frequency of unsupported monolayer graphene to be
5.4 cm−1 GPa−1, very close to that of graphite (4.7 cm−1 GPa−1), contrary to the previous value for
supported graphene. Our results imply that monolayer graphene has similar in-plane and out-of-plane
stiffnesses, and anharmonicities to graphite.

KeyWords Plus:

DOI: 10.1103/PhysRevLett.123.135501

Author(s): C. Martín-Sánchez, G. González-Rubio, P. Mulvaney, A. Guerrero-Martínez, L. M. Liz-Marzán, and F. Rodríguez

Source: Journal of Physical Chemistry Letters

Volume: 10 (7)    Páginas: 1587-1593

Abstract: The effects of hydrostatic pressure on the surface plasmon resonances (SPRs) of aqueous dispersions of monodisperse gold nanorods (AuNRs) were determined up to 9 GPa. The ultranarrow longitudinal SPR band of monodisperse AuNRs allows us to monitor a gradual red shift with pressure, which shows abrupt jumps at the liquid to ice phase VI and ice phase VII transitions. Despite solidifying at low pressure (∼1.8 GPa), water displays a regime of quasi-hydrostaticity in said phases VI and VII, up to ca. 5 GPa. Above this pressure, nonhydrostatic effects manifest themselves through broadening of the SPR bands, but barely any effect is observed on the position of the surface plasmon mode. The variation in the SPR peak wavelength with pressure allowed us to determine the pressure dependence of the refractive index of water. Unlike Brillouin scattering or interferometric techniques, this plasmon-spectroscopy-based method leads to a more direct determination of the refractive index, which is well described empirically by Murnaghan-type equations in the three explored phases. We report herein the obtained analytical functions providing the pressure dependence of refractive index in the liquid, ice VI, and ice VII phases of water.

KeyWords Plus: Liquids, optical properties, phisical and chemical process, surface plasmon resonance

DOI: https://doi.org/10.1021/acs.jpclett.9b00636

Author(s): V. Gutiérrez-Cano, F. Rodríguez, J. A. González, and R. Valiente

Source: PapThe Journal of Physical Chemistry C

Volume: 123 (49)    Páginas: 29818-29828

Abstract: The search of material hosts being able to incorporate Er³⁺ impurities with a thermally stable structure and high melting temperature is priority in optical thermometry. In this work, we report on the structural and spectroscopic characterization of Er³⁺–doped and Yb³⁺/Er³⁺–co-doped LaGdO₃ nanocrystals synthesized via the sol-gel Pechini method. X-ray diffraction (XRD) and Raman spectroscopy unequivocally show that the synthesis method provides nanocrystals with a single-phase B-type monoclinic structure (space group: C2/m). Intensity decay curves I(t) were measured to investigate the efficiency of upconversion processes yielding green emission. We showed that an energy transfer upconversion (ETU) process involving Yb–Er pairs governs visible emission upon NIR excitation. The temperature dependence of the thermalized green luminescence at 525 nm (²H11/2→⁴I_15/2) and 549 nm (⁴S_3/2→⁴I_15/2) was checked for thermometric applications in the RT–900 K temperature range. We demonstrate that the B-type monoclinic phase of LaGdO₃ is stable from low temperature up to 900 K. Doped with Er³⁺, it shows suitable thermometer capabilities with a maximum sensitivity of S = 4.3·10⁻³ K⁻¹ at 554 K, and a relative sensitivity decreasing from its maximum value at 0 K to S_R = 1.2 · 10⁻² K⁻¹ at 298 K. The results suggest that LaGdO₃ in its B-type monoclinic phase is a promising material as a wide-range temperature sensor, without any further surface protection.

KeyWords Plus: Luminescence, Power, Materials, Lasers, Ions

 DOI: http://dx.doi.org/10.4279/PIP.110004

Author(s): E. Jara, J. A. Barreda-Argüeso, J. González, R. Valiente and F. Rodríguez

Source: Papers in Physics (Focus Series on High Pressure Semiconductor and Superconductor Physics)

Volume: 11    Issue: 110004 

Abstract: We have investigated the crystal structure of Cs2CuCl4 in the 0-20 GPa range as a function of pressure and how pressure affects its electronic properties by means of optical absorption spectroscopy. In particular, we focused on the electronic properties in the low-pressure Pnma phase, which are mainly related to the tetrahedral CuClp units distorted by the Jahn-Teller effect. This study provides a complete characterization of the electronic structure of Cs2CuCl4 in the Pmna phase as a function of the cell volume and the Cu-Cl bond length, Rc-u-ci. Interestingly, the opposite shift of the charge-transfer band-gap and the Cu1 4+ d-d crystal-field band shift with pressure are responsible for the strong piezochromism of Cs2CuCl4. We have also explored the high-pressure structure of Cs2CuCl4 above 4.9 GPa yielding structural transformations that are probably associated with a change of coordination around Cu2+. Since the high-pressure phase appears largely amorphized, any structural information from X-ray diffraction is ruled out. We use electronic probes to get structural information of the high-pressure phase.

KeyWords Plus: Cs2CuCl4; electronic structure; high pressure; band gap.

 DOI: http://dx.doi.org/10.4279/PIP.110004

Author(s): Camino Martín-Sánchez, J. Antonio Barreda-Argüeso, Susanne Seibt, Paul Mulvaney, and Fernando Rodríguez

Source: ACS Nano

Volume: 13 (1)    Páginas: 498-504  

 DOI: 10.1021/acsnano.8b07104

Abstract: The The surface plasmon resonances of gold nanospheres and nanorods have been measured as a function of hydrostatic pressure up to 17 GPa in methanol− ethanol 4:1 solvent and up to 10 GPa in

paraffi n. Both the sphere resonance and the longitudinal rod resonance exhibit redshifts, whereas the transverse rod mode shows an extremely weak redshift or blueshift depending on the nanorod aspect ratio. Solidifi cation of the solvent around 11 GPa causes some aggregation of the particles, readily identifi ed through broadening of the surface plasmon band and further redshifting. Spectra collected during loading and unloading cycles exhibit only minimal hysteresis if the pressure remains below 11 GPa. The surface plasmon shifts are the result of two competing eff ects. Compression of the conduction electrons in the metals increases the bulk plasma frequency, which causes a blueshift. However, the increase in the solvent density under hydrostatic load leads to an increase in the solvent refractive index, which in turn leads to a redshift. We fi nd that after accounting for the solvent contribution, we can spectroscopically determine the bulk modulus of the gold nanoparticles with a precision of 10%. The value obtained of K0  = 190 GPa is signifi cantly higher than the value for bulk gold (167 GPa). Furthermore, we show that pressure-induced solidifi cation causes a signifi cant broadening and anomalous shift of the surface plasmon band that we attribute to aggregation and nanorod deformation. 

KeyWords Plus: Gold nanoparticles, hydrostatic pressure, surface plasmon resonance, optical absorption, bulk modulus, refractive index

Author(s): Vanessa Gutiérrez-Cano, Fernando Rodríguez, Jesús A. González, and Vanesa Díaz

Source: Thin Solid Films.

Volume: 648   Páginas: 8–11   DOI: doi.org/10.1016/j.tsf.2017.12.023 

Abstract:

This work reports an alternative method for determining thermal degradation of graphite-based thin film hydrophobic coatings using the D and G graphite vibrations as a probe. We show that the Raman spectrum of the coating as a function of temperature is an efficient probe for detecting changes in the surface properties. Both D and G bands of graphite are sensitive to changes of the coating matrix induced by temperature. We apply the Raman technique to correlate the changes of D and G vibrations with those found by different experimental methods such as contact angle measurements and thermogravimetric analysis to characterize organosilane hydrophobic coatings.

KeyWords Plus: Raman spectroscopy, Carbon-based materials, Graphite, Thermal degradation, Hydrophobic coatings, Coatings

Author(s): Silvia Blanco, Fernando Aguado, Jesús A. González, and Fernando Rodriguez

Source: J. Phys.: Condens. Matter

Volume: 30   Páginas: 304002 (9pp) DOI: 10.1088/1361-648X/aacb1a

Abstract:

Here we report a structural characterization study of glass-ceramics as a function of pressure by Raman spectroscopy. We selected a glass-ceramics of Li₂O–Al₂O₃–nSiO₂ (n  ≈  8) doped with ZrO₂ and TiO₂. This composition induces slight structural modification in Si–O and Si–O–Si bonds in the glass matrix that are crucial to stabilize metastable hexagonal SiO₂ (β-quartz) solid-solution nano crystals corresponding to γ-LiAlSi₂O₆. This structure yields a more compact β-quartz-type phase that eventually stabilizes it in wider pressure range than pure SiO₂. Raman spectroscopy allows to unravel at least two pressure-induced structural phase transitions at about 6 and 15 GPa that could not be previously revealed by x-ray diffraction. We show that the phase-transition sequence can tentatively be described in terms of SiO₂-type structure as: β-quartz  →  Coesite I  →  Coesite II. The measured transition pressures are consistent with the larger cell volume attained in the γ-LiAlSi₂O₆ yielding a wider metastability pressure range of the β-quartz-type phase.

Author(s): J. Ruiz-Fuertes, D. Martínez-García, T. Marqueño, D. Errandonea, S. G. MacLeod, T. Bernert, E. Haussühl, D. Santamaría-Pérez, J. Ibáñez, A. Mallavarapu, S. N. Achary, C. Popescu, and M. Bettinelli.

Source: Inorg. Chem.

Volume: 57       Páginas: 14005

 DOI: 10.1021/acs.inorgchem.8b01808

Abstract: The zircon to scheelite phase boundary of ErVO₄ has been studied by high-pressure and high-temperature powder and single-crystal X-ray diffraction. This study has allowed us to delimit the best synthesis conditions of its scheelite-type phase, determine the ambient-temperature equation of state of the zircon and scheelite-type structures, and obtain the thermal equation of state of the zircon-type polymorph. The results obtained with powder samples indicate that zircon-type ErVO₄ transforms to scheelite at 8.2 GPa and 293 K and at 7.5 GPa and 693 K. The analyses yield bulk moduli K₀ of 158(13) GPa for the zircon phase and 158(17) GPa for the scheelite phase, with a temperature derivative of dK₀/dT = −[3.8(2)] × 10^–3 GPa K^–1 and a volumetric thermal expansion of α₀ = [0.9(2)] × 10^–5 K^–1 for the zircon phase according to the Berman model. The results are compared with those of other zircon-type vanadates, raising the need for careful experiments with highly crystalline scheelite to obtain reliable bulk moduli of this phase. Finally, we have performed single-crystal diffraction experiments from 110 to 395 K, and the obtained volumetric thermal expansion (α₀) for zircon-type ErVO₄ in the 300–395 K range is [1.4(2)] × 10^–5 K^–1, in good agreement with previous data and with our experimental value given from the thermal equation of state fit within the limits of uncertainty.

Author(s): S. Anzellini, D. Errandonea, S. G. MacLeod, P. Botella, D. Daisenberger, J. M. De’Ath, J. González-Platas, J. Ibáñez, M. I. McMahon, K. A. Munro, C. Popescu, J. Ruiz-Fuertes, and C. W. Wilson

Source: Phys. Rev. Materials

Volume: 2    Páginas: 083608

DOI: https://doi.org/10.1103/PhysRevMaterials.2.083608

Abstract:

Resistively heated diamond-anvil cells have been used together with synchrotron x-ray diffraction to investigate the phase diagram of calcium up to 50 GPa and 800 K. The phase boundaries between the Ca-I (fcc), Ca-II (bcc), and Ca-III (simple cubic, sc) phases have been determined at these pressure-temperature conditions, and the ambient temperature equation of state has been generated. The equation of state parameters at ambient temperature have been determined from the experimental compression curve of the observed phases by using third-order Birch-Murnaghan and Vinet equations. A thermal equation of state was also determined for Ca-I and Ca-II by combining the room-temperature Birch-Murnaghan equation of state with a Berman-type thermal expansion model.

Author(s): D. Errandonea, S. G. MacLeod, J. Ruiz-Fuertes, L. Burakovsky, M. I. McMahon, C. W. Wilson, J. Ibáñez, D. Daisenberger, and C. Popescu

Source: J. Phys.: Condens. Matter

Volume: 30       Páginas: 295402

 DOI: 10.1088/1361-648X/aacac0

Abstract:

The phase diagram of zinc (Zn) has been explored up to 140 GPa and 6000 K, by combining optical observations, x-ray diffraction, and ab initio calculations. In the pressure range covered by this study, Zn is found to retain a hexagonal close-packed (hcp) crystal symmetry up to the melting temperature. The known decrease of the axial ratio (c/a) of the hcp phase of Zn under compression is observed in x-ray diffraction experiments from 300 K up to the melting temperature. The pressure at which c/a reaches  (≈10 GPa) is slightly affected by temperature. When this axial ratio is reached, we observed that single crystals of Zn, formed at high temperature, break into multiple poly-crystals. In addition, a noticeable change in the pressure dependence of c/a takes place at the same pressure. Both phenomena could be caused by an isomorphic second-order phase transition induced by pressure in Zn. The reported melt curve extends previous results from 24 to 135 GPa. The pressure dependence obtained for the melting temperature is accurately described up to 135 GPa by using a Simon–Glatzel equation: , where P is the pressure in GPa. The determined melt curve agrees with previous low-pressure studies and with shock-wave experiments, with a melting temperature of 5060(30) K at 135 GPa. Finally, a thermal equation of state is reported, which at room-temperature agrees with the literature.KeyWords Plus: zinc, x-ray diffraction, ab initio calculations, high pressure, high temperature, phase transition, melting

Author(s): J. Herrero-Martín, J. Ruiz-Fuertes, T. Bernert, M. Koch-Müller, E. Haussühl, and J. L. García-Muñoz.

Source: Phys. Rev. B

Volume: 97    Páginas: 235129

 DOI: doi.org/10.1103/PhysRevB.97.235129

Abstract:

The ferroelectric and magnetic phases of the double perovskite CaMnTi2O6 with A-site order have been investigated by soft x-ray absorption and magnetic circular dichroism. All spectra point to a very ionic state of divalent Mn and tetravalent Ti atoms. The effects of the crystal field produced by O ligands around tetravalent titanium and the dissimilar Mn1 and Mn2 sites were investigated. Both the so-called square-planar and the octahedrally coordinated Mn sites spectroscopically contribute in a rather similar way, with little influence by the oxygen environment. Multiplet calculations suggest a small O2p−Ti3d charge-transfer component in the FE phase. Magnetic symmetry calculations were performed to determine probable configurations of Mn spins compatible with the acentric P42mcstructure and, in combination with the computational magnetic results in Inorg. Chem. 56, 11854 (2017), we have identified the P4′2m′c as the most likely magnetic space group keeping invariant the unit cell below TN. This symmetry forces the sign of the magnetic coupling along the Mn columns parallel to c to reverse with respect to the coupling between neighboring columns. Below TN, the dichroic magnetization loops at the MnL3 edge confirm the absence of spontaneous ferromagnetism, although a very small field-induced spin polarization was detected in the sample.

Author(s): T. Marqueño, D. Santamaría-Pérez, J. Ruiz-Fuertes, R. Chuliá-Jordán, J. L. Jordá, F. Rey, C. MaGuire, A. Kavner, S. MacLeod, D. Daisenberger, C. Popescu, P. Rodríguez-Hernández, and A. Muñoz

Source: Inorg. Chem

Volume: 57    Páginas: 6447

 DOI: 10.1021/acs.inorgchem.8b00523

Abstract:

We report the formation of an ultrahigh CO₂-loaded pure-SiO₂ silicalite-1 structure at high pressure (0.7 GPa) from the interaction of empty zeolite and fluid CO₂ medium. The CO₂-filled structure was characterized in situ by means of synchrotron powder X-ray diffraction. Rietveld refinements and Fourier recycling allowed the location of 16 guest carbon dioxide molecules per unit cell within the straight and sinusoidal channels of the porous framework to be analyzed. The complete filling of pores by CO₂ molecules favors structural stability under compression, avoiding pressure-induced amorphization below 20 GPa, and significantly reduces the compressibility of the system compared to that of the parental empty one. The structure of CO₂-loaded silicalite-1 was also monitored at high pressures and temperatures, and its thermal expansivity was estimated.

Author(s): A. L. J. Pereira, D. Santamaría-Pérez, J. Ruiz-Fuertes, F. J. Manjón, V. P. Cuenca-Gotor, R. Vilaplana, O. Gomis, C. Popescu, A. Muñoz, P. Rodríguez-Hernández, A. Segura, L. Gracia, A. Beltrán, P. Ruleova, C. Drasar, and J. A. Sans

Source: J. Phys. Chem. C

Volume: 122  Páginas: 8853

 DOI: 10.1021/acs.jpcc.8b02194

Abstract: We report a joint experimental and theoretical study of the structural, vibrational, elastic, optical, and electronic properties of the layered high-mobility semiconductor Bi₂O₂Se at high pressure. A good agreement between experiments and ab initio calculations is observed for the equation of state, the pressure coefficients of the Raman-active modes and the bandgap of the material. In particular, a detailed description of the vibrational properties is provided. Unlike other Sillén-type compounds which undergo a tetragonal to collapsed tetragonal pressure-induced phase transition at relatively low pressures, Bi₂O₂Se shows a remarkable structural stability up to 30 GPa; however, our results indicate that this compound exhibits considerable electronic changes around 4 GPa, likely related to the progressive shortening and hardening of the long and weak Bi–Se bonds linking the Bi₂O₂ and Se atomic layers. Variations of the structural, vibrational, and electronic properties induced by these electronic changes are discussed.

Author(s): D. Zimmer, J. Ruiz-Fuertes, W. Morgenroth, A. Friedrich, L. Bayarjargal, E. Haussühl, D. Santamaría-Pérez, S. Frischkorn, V. Milman, and B. Winkler

Source: Phys. Rev

Volume: 97    Páginas: 134111

 DOI:              10.1103/PhysRevB.97.134111

Abstract: The high-pressure behavior of monoclinic (P21/c) α-chalcocite, Cu2S, was investigated at ambient temperature by single-crystal x-ray diffraction, electrical resistance measurements, and optical absorption spectroscopy up to 16 GPa. The experiments were complemented by density-functional-theory-based calculations. Single-crystal x-ray diffraction data show that monoclinic α-chalcocite undergoes two pressure-induced first-order phase transitions at ∼3.1 and ∼7.1 GPa. The crystal structure of the first high-pressure polymorph, HP1, was solved and refined in space group P21/c with a=10.312(4)Å, b=6.737(3)Å, c=7.305(1)Å, and β=100.17(2)∘ at 6.2(3) GPa. The crystal structure of the second high-pressure polymorph, HP2, was solved and refined in space group P21/c with a=6.731(4)Å, b=6.689(2)Å, c=6.967(8)Å, and β=93.18(3)∘ at 7.9(4) GPa. Electrical resistance measurements upon compression and optical absorption experiments upon decompression show that the structural changes in α-chalcocite are accompanied by changes of the electrical and optical properties. Upon pressure release, the band gap Eg of α-chalcocite (1.24 eV at ambient conditions) widens across the first structural phase transition, going from 1.24 eV at 2.2 GPa (α-chalcocite) to 1.35 eV at 2.6 GPa (HP1), and closes significantly across the second phase transition, going from 1.32 eV at 4.4 GPa (HP1) to 0.87 eV at 4.9 GPa (HP2). The electrical resistance shows similar behavior: its highest value is for the first high-pressure polymorph (HP1), and its lowest value is for the second high-pressure polymorph (HP2) of α-chalcocite. These results are interpreted on the basis of calculated electronic band structures.

Author(s): D. Errandonea, O. Gomis, P. Rodríguez-Hernández, A. Muñoz, J. Ruiz-Fuertes, M. Gupta, S. N. Achary, A. Hirsch, F. J. Manjón, L. Peters, G. Roth, A. K. Tyagi, and M. Bettinelli

Source: J. Phys.: Condens. Matter

Volume: 30    Páginas: 065401

 DOI: doi.org/10.1088/1361-648X/aaa20d

Abstract:

Monazite-type BiPO₄, LaPO4, and PrPO4 have been studied under high pressure by
ab initio simulations and Raman spectroscopy measurements in the pressure range of stability of the monazite structure. A good agreement between experimental and theoretical Raman- active mode frequencies and pressure coefficients has been found which has allowed us to discuss the nature of the Raman-active modes. Besides, calculations have provided us with information on how the crystal structure is modified by pressure. This information has allowed us to determine the equation of state and the isothermal compressibility tensor of the four studied compounds. In addition, the information obtained on the polyhedral compressibility has been used to explain the anisotropic axial compressibility and the bulk compressibility of monazite phosphates. Finally, we have carried out a systematic discussion on the high-pressure behavior of the four studied phosphates in comparison to results of previous studies.

Keywords: monazite, orthophosphate, high pressure, equation of state, Raman, Ab initio calculations

Author(s): D. Errandonea and J. Ruiz-Fuertes

Source: Crystals

Volume: 8      Páginas: 71

 DOI: doi.org/10.3390/cryst8020071

Abstract:

In this article we review the advances that have been made on the understanding of the high-pressure structural, vibrational, and electronic properties of wolframite-type oxides since the first works in the early 1990s. Mainly tungstates, which are the best known wolframites, but also tantalates and niobates, with an isomorphic ambientpressure wolframite structure, have been included in this review. Apart from estimating the bulk moduli of all known wolframites; the cation-oxygen bond distances and their change with pressure have been correlated with their compressibility. The composition variations of all wolframites have been employed to understand their different structural phase transitions to post-wolframite structures as a response to high pressure. The number of Raman modes and band gap energy changes have been also analyzed in the basis of these compositional differences. The reviewed results are relevant for both fundamental science and for the development of wolframites as scintillating detectors. The possible next research venues of wolframites have also been evaluated.Keywords: wolframite, high-pressure, phase transitions, crystal structure, phonons, band structure.

Author(s): J. Ruiz-Fuertes, O. Gomis, A. Segura, M. Bettinelli, M. Burianek, and M. Mühlberg

Source: Appl. Phys. Lett

Volume: 112  Páginas: 042901

 DOI: https://doi.org/10.1063/1.5012111

Abstract:

In this letter, we have investigated the electronic structure of AxBa1–xNb2O6 relaxor ferroelectrics on the basis of optical absorption spectroscopy in unpoled single crystals with A = Sr and Ca under high pressure. The direct character of the fundamental transition could be established by fitting Urbach’s rule to the photon energy dependence of the absorption edge yielding bandgaps of 3.44(1) eV and 3.57(1) eV for A = Sr and Ca, respectively. The light scattering by ferroelectric domains in the pre-edge spectral range has been studied as a function of composition and pressure. After confirming with x-ray diffraction the occurrence of the previously observed ferroelectric to paraelelectric phase transition at 4 GPa, the light scattering produced by micro- and nano-ferroelectric domains at 3.3 eV in Ca0.28Ba0.72Nb2O6 has been probed. The direct bandgap remains virtually constant under compression with a drop of only 0.01 eV around the phase transition. Interestingly, we have also found that light scattering by the polar nanoregions in the paraelectric phase is comparable to the dispersion due to ferroelectric microdomains in the ferroelectric state. Finally, we have obtained that the bulk modulus of the ferroelectric phase of Ca0.28Ba0.72Nb2O6 is B0 = 222(9) GPa.

Author(s): J. Marquina, J. Martín, J. Luengo, F. Vera, L. Roa, G.E. Delgado, F. Rodríguez, C. Renero-Lecuna, R. Valiente, and J. González

Source: Revista Mexicana de Física

Volume: 63 (1)   Páginas: 32-39 

Abstract:

We report the results of the Rietveld refinement, photoluminescence and Raman spectroscopy of Mn-doped ZnO ceramic pellets. Rietveld refinement shows that samples crystallize in the wurtzite structure and for the Mn-doped sample indicates that the Mn atoms substitute the Zn tetrahedral crystallographic sites in the ZnO host lattice. The emission and absorption spectra of Mn-doped ZnO have been investigated in the Visible-UV region and the data have been interpreted in terms of the wurtzite ZnO electronic structure. Two broad bands, one due to superposition between donor bound excitons (DX) and free excitons (FX) and other due free-to-bond excitonic recombination (FB) dominates the low-temperature photoluminescence spectra of Mn-doped ZnO bulk. In the Raman spectrum, an extra mode at 520 cm-1 has been observed in agreement with earlier works, and it is an indicator for the incorporation of Mn+2 ions into the ZnO host matrix since it is not is observed in ZnO pristine.

Keywords: DMS; Photoluminescence; Raman spectroscopy; ZnO

Author(s): F. Rodríguez

Source: Cap. del libro Correlations in Condensed Matter under Extreme Conditions, Ed. Springer International Publishing

Páginas: 3-15

Abstract: Jahn-Teller (JT) transition-metal ions like Cu\(^{2+}\) (\(d^9\)) or Mn\(^{3+}\) (\(d^4\)) in octahedral coordination exhibit larger distortions than non-JT ions like Mn\(^{2+}\) or Fe\(^{3+}\) (both \(d^5\)) in oxides and halides with perovskite type structure. Their mutual interactions eventually determine the type of distortion and the way they couple each other, being at the core of some relevant physical properties. When a JT ion is introduced as an impurity in an octahedral or nearly octahedral site, it provokes a low-symmetry lattice distortion as a consequence of the instability associated with the electronic ground state degeneracy, \(e_g (x^2 -y^2, 3z^2 -r^2 )\). The distortion degree, \(\rho \), depends mainly on the electron-ion coupling interaction related to the \(E\otimes e\) JT effect, and it is modulated by the host crystal structure. This scenario explains for example why Cu\(^{2+}\) induces a large distortion of the CuF\(_6\) octahedron, when Cu\(^{2+}\) replaces Zn\(^{2+}\) either in the perovskite KZnF\(_3\) or in the layered perovskite K\(_2\)ZnF\(_4\). However, there is a long debate about whether the splitting, \(\varDelta _e\) of the \(O_h\) \(e_g (x^2 -y^2, 3z^2 -r^2 )\) orbitals into \(a_{1g}\) and \(b_{1g}\) is proportional to \(\rho \) or \(\varDelta _e\) contains additional contributions from the rest-of-the-lattice (crystal anisotropy) aside \(\rho \). Elucidation of this controversy is important in order to establish structural correlations between \(\varDelta _e\) and \(\rho \), for an eventual local structure determination of JT impurities from optical spectroscopy. Recent studies on K\(_2\)ZnF\(_4\):Cu\(^{2+}\) report different views of this problem. Here we show that \(\varDelta _e\) scales linearly with \(\rho \). High pressure experiments and JT-ion compound series of different dimensionality give support for the proposed scenario and provide structural correlations relating \(\rho \) and \(\varDelta _e\) in Cu\(^{2+}\) and Mn\(^{3+}\) systems.

Author(s): L. Nataf, F. Aguado, I. Hernández, R. Valiente, J. González, M.N. Sanz-Ortiz,  H. Wilhelm, A.P. Jephcoat, F. Baudelet, and F. Rodríguez

Source: Physical Review B

Volume: 95 (1)   Páginas: 014110

DOI: 10.1103/PhysRevB.95.014110

Abstract: This work investigates the high-pressure structure of Cs2CoCl4 and how it affects the electronic and vibrational properties using optical absorption, Raman spectroscopy, x-ray diffraction, and x-ray absorption in the 0–15 GPa range. In particular, we focus on the electronic and local structures of Co2+ , since compression of Cs2 CoCl4 yields structural transformations associated with change of coordination around Co2+, which are eventually responsible for the intense piezochromism at 7 GPa. This study provides a complete characterization of the electronic and vibrational structures of Cs2CoCl4 in the Pnma phase as a function of the cell volume and the local CoCl4 bond length, RCo-Cl, as well as its corresponding equation of state. In addition, our interest is to elucidate whether the phase transition undergone by Cs2CoCl4 at 7 GPa leads to a perovskite-layer-type structure where Co2+ is sixfold coordinated, decomposes into CsCl + CsCoCl3 , or it involves an unknown phase with different coordination sites for Co2+. We show that Co2+ is sixfold coordinated in the high-pressure phase. The analysis of optical spectra and x-ray diffraction data suggests the formation of an interconnected structure of exchange-coupled Co2+ through edge-sharing octahedra at high pressure.

Author(s): F. Rodríguez

Source: Inorganic Chemistry

Volume: 56 (4)   Páginas: 2019-2036

DOI: 10.1021/acs.inorgchem.6b02677

Abstract:

A_nMX_m perovskites with Jahn−Teller M ions exhibit an ample variety of physical phenomena. The local structure of MX₆ octahedra and the way they couple with each other are at the core of their properties. Correlations between structure and d-orbital electronic splitting in Cu²⁺ and Mn³⁺ show that MX₆ distortions scale linearly with the splitting Δ_e of e_g(x² − y², 3z² − r²) orbitals. We apply such correlations to determine the low-symmetry distortion of Cu²⁺ impurities in KZnF₃ and K₂ZnF₄ by optical spectroscopy.

Author(s): R. Martín-Rodríguez, R. Valiente, F. Aguado, and A.C. Perdigón

Source: Journal of Materials Chemistry C

Volume: 5 (39)   Páginas: 10360-10368

DOI: 10.1021/acs.inorgchem.6b02677

Abstract:

A_nMX_m perovskites with Jahn−Teller M ions exhibit an ample variety of physical phenomena. The local structure of MX₆ octahedra and the way they couple with each other are at the core of their properties. Correlations between structure and d-orbital electronic splitting in Cu²⁺ and Mn³⁺ show that MX₆ distortions scale linearly with the splitting Δ_e of e_g(x² − y², 3z² − r²) orbitals. We apply such correlations to determine the low-symmetry distortion of Cu²⁺ impurities in KZnF₃ and K₂ZnF₄ by optical spectroscopy.

Author(s): Jack Binns, Garry J McIntyre, José A Barreda-Argüeso, Jesús González, Fernando Aguado, Fernando Rodríguez, Rafael Valiente, and Simon Parsons

Source: Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials

Volume: 73 Páginas: 844-855

DOI: 10.1107/S2052520617006412

Abstract:

The phase transition sequences of two members of the tetramethylammonium tetrachlorometallate(III) family of hybrid organic-inorganic salts have been determined and structurally characterized as a function of temperature for the first time. Unusually, a reduction in point-group symmetry with increasing temperature until reaching a cubic prototype phase is observed. Two additional intermediate phases are observed for Fe3+. First-principles calculations and the presence of short Cl…Cl contacts for Ga3+ suggest the [GaCl4]- anion to be conformationally hindered due to stronger lone-pair-σ-hole interactions. The conformationally more flexible Fe3+ structures show sublattice melting with the onset of rotational disorder in the [NMe4]+ cations occurring 40 K below the corresponding onset of rotational disorder in the [FeCl4]- sublattice.

KEYWORDS: crystal packing; intermolecular interactions; phase transitions

Author(s): J.A. Barreda-Argüeso, F. Aguado, J. González, R. Valiente, L. Nataf, M.N. Sanz-Ortiz, and F. Rodríguez

Source: Journal of Physics: Conference Series

Volume: 950 Páginas: 042016

DOI:  10.1088/1742-6596/950/4/042016

Abstract: This work reports an optical absorption study on CoF₂ under high-pressure conditions in the 0-80 GPa range. A recent structural study1 performed on the pressure-induced phase-transition sequence of CoF₂ reveals that this compound undergoes a structural transformation associated with a change of Co²⁺ coordination from 6 (rutile type) to 8 (fluorite type) at 15 GPa, what implies a profound change in the Co²⁺ electronic ground state associated with the inversion of d orbitals from (t_2g 5 e _g 2) to (e_g 4t_2g 3). This allows us to explore the Co²⁺ electronic structure, and hence 10Dq, as a function of the density and the Co-F bond length in both CoF₆ (rutile phase) and CoF₈ (fluorite phase) coordinations. The results are compared with those obtained in KCoF₃ (perovskite) in a wide range of Co-F distances (from 2.04 to 1.83 Å).

Author(s): H. Lu, Y. Peng, H. Ye, X. Cui, J. Hu, H. Gu, A. N. Khlobystov, M. A. Green, P.J. Blower, P.B. Wyatt, W.P. Gillin and I. Hernández

Source: Scientific Reports 

Volume: 7 Páginas: 5066

DOI:  doi.org/10.1038/s41598-017-05350-9

Abstract:

Infra-red emission (980 nm) of sub 10 nm Yb3+-doped NaYF4 nanoparticles has been sensitized through the excitation of 2-hydroxyperfluoroanthraquinone chromophore (1,2,3,4,5,6,7-heptafluro-8-hydroxyanthracene-9,10-dione) functionalizing the nanoparticle surface. The sensitization is achieved with a broad range of visible light excitation (400–600 nm). The overall near infra-red (NIR) emission intensity of Yb3+ ions is increased by a factor 300 as a result of the broad and strong absorption of the chromophore compared with ytterbium’s intrinsic absorption. Besides the Yb3+ NIR emission, the hybrid composite shows organic chromophore-based visible emission in the orange-red region of the spectrum. We observe the energy migration process from the sensitized Yb3+ ions at the surface to those in the core of the particle using time-resolved optical spectroscopy. This highlights that the local environments for emitting Yb3+ ions at the surface and center of the nanoparticle are not identical, which causes important differences in the NIR emission dynamics. Based on the understanding of these processes, we suggest a simple strategy to control and modulate the decay time of the functionalized Yb3+-doped nanoparticles over a relatively large range by changing physical or chemical parameters in this model system.

Author(s): J. Marquina, J. Martín, J. Luengo, F. Vera, L. Roa, G.E. Delgado, F. Rodríguez, C. Renero-Lecuna, R. Valiente and J. González

Source: Revista Mexicana de Física

Volume: 63 Páginas: 32-39

ISSN 0035-001X

Abstract:

We report the results of the Rietveld refinement, photoluminescence and Raman spectroscopy of Mn-doped ZnO ceramic pellets. Rietveld refinement shows that samples crystallize in the wurtzite structure and for the Mn-doped sample indicates that the Mn atoms substitute the Zn tetrahedral crystallographic sites in the ZnO host lattice. The emission and absorption spectra of Mn-doped ZnO have been investigated in the Visible-UV region and the data have been interpreted in terms of the wurtzite ZnO electronic structure. Two broad bands, one due to superposition between donor bound excitons (DX) and free excitons (FX) and other due free-to-bond excitonic recombination (FB) dominates the low-temperature photoluminescence spectra of Mn-doped ZnO bulk. In the Raman spectrum, an extra mode at ∼520 cm-1 has been observed in agreement with earlier works, and it is an indicator for the incorporation of Mn+2 ions into the ZnO host matrix since it is not is observed in ZnO pristine.

Author(s): R Martín-Rodríguez, R Valiente, F Aguado, AC Perdigón

Source: Journal of Materials Chemistry C

Volume: 5 Páginas: 10360-10368

DOI:10.1039/C7TC01818E

Abstract:

Incorporation of lanthanide ions in synthetic clay minerals is a promising approach to combine the efficient sharp-line emission of lanthanides with the unique structural stability and high adsorption capacity of high-charge micas. These aluminosilicate hosts are synthetic exchanger layered-clays which have demonstrated a unique selectivity in heavy metal and radionuclide capture. A phosphor based on the emission of Eu3+ ions homogeneously distributed in the dispersing inorganic-host high-charge mica Na2[Mg6]VIII[Si6Al2]IVO20F4 is reported. Eu3+ cations act as an ideal luminescent probe to study the physical–chemical adsorption mechanisms of the contaminants in the aluminosilicate. Despite the forbidden nature of the f–f transition involved, well-resolved Eu3+ luminescence is observed from both the 5D0 and the 5D1 Eu3+ states in the high-charge mica, and, in contrast to previous results, the use of an antenna for light absorption and transfer to the lanthanide is not required. The fluorinated nature and the absence of iron impurities, together with the homogeneous dispersion of lanthanide ions provided by Al3+, makes the high-charge mica a biocompatible host as an optical sensor or drug delivery material. Further evidence of the homogeneous distribution and isolation of Eu3+ cations in the hexagonal cavities of the mica is provided by spectroscopic luminescence and lifetime measurements.

Author(s): Susana Gómez-Salces, Jose Antonio Barreda-Argüeso, Rafael Valiente, Fernando Rodríguez

Publication date: 2016

Source: Journal of Materials Chemistry C                       Volumen:  4 Issue:            38       Pag.: 9021-9026

DOI: 10.1039/C6TC01408A

Abstract:

This work investigates the energy transfer from Ce³⁺ to Mn²⁺ in high transmission glass (HTG) doped with CeO₂ and MnO through time-resolved spectroscopy to transform the solar spectrum into a more efficient red-enhanced spectrum for traditional Si-based solar cells. We show that both Mn³⁺ and Mn²⁺ and Ce³⁺ and Ce⁴⁺ centres are formed in HTG through their absorption and emission/excitation spectra. Interestingly, Ce³⁺ excitation at 320 nm yields both Ce³⁺ (400 nm) and Mn²⁺ (530 nm) emissions for doping concentrations of 0.1–1% Ce³⁺ and 0.1% Mn²⁺. The energy transfer process in HTG is noteworthy since it enhances the capability for blue-to-red light transformation, which is important for a concentrator in photovoltaic applications. This work analyses the non-radiative vs. radiative Ce³⁺ → Mn²⁺ energy transfer process in this optically enriched HTG. In the explored doping range we show that energy transfer is purely radiative

Author(s): GV. Vázquez, R. Valiente, S. Gómez-Salces, E. Flores-Romero, J. Rickards, R. Trejo-Luna

Publication date: 2016/5/31

Source: Optics & Laser Technology                 Volumen:  79   Issue: 8     Pag.: 132

DOI: 10.1016/j.optlastec.2015.12.002

Abstract:

Channel waveguides were fabricated by carbon implantation in soda lime glass samples doped with Er ³⁺ and Yb ³⁺, exhibiting good confinement and both monomode and multimode behaviour at 633 nm. Excitation at near infrared (NIR) and ultraviolet (UV) spectral ranges were used in order to obtain anti-Stokes (upconversion) and Stokes (downshift) emission in the visible range, respectively. The characteristic green and red bands of Er ³⁺ transitions were observed, showing the potential of Yb ³⁺ and Er ³⁺ co- …

Author(s): J Antonio Barreda-Argüeso, Fernando Aguado, Jesús González, Rafael Valiente, Lucie Nataf, Marta N Sanz-Ortiz, Fernando Rodríguez

Publication date: 2016/8/17

Source:  Journal of physical chemistry. C       Volumen:  120   Issue: 33            Pag.: 18788

DOI: 10.1021/acs.jpcc.6b06132

Abstract:

Crystal field theory (CFT) predicts that crystal field acting on an transition-metal (TM) ion complex of cubic symmetry varies as R^–5, where R is the TM-ligand distance. Yet simple and old-fashioned, CFT is used extensively since it provides excellent results in most TM ion-bearing systems, although no direct and thorough validation has been provided so far. Here we investigate the evolution of the electronic and crystal structures of two archetypal Co²⁺ compounds by optical absorption and X-ray diffraction under high pressure. Both the electronic excited states and crystal-field splitting, Δ = 10Dq, between 3d(e_g + t_2g) orbitals of Co²⁺ as a function of volume, V, and Co–F bond length, R, in 6-fold octahedral (oct) and 8-fold hexahedral (cub) coordination in compressed CoF₂ have been analyzed. We demonstrated that Δ scales with R in both coordinations as R^–n, with n close to 5 in agreement with CFT predictions. The pressure-induced rutile to fluorite structural phase transition at 15 GPa in CoF₂ is associated with an increase of R due to the 6 → 8 coordination change. The experimental Δ(oct)/ Δ(cub) = −1.10 for the same R-values is close to −9/8, in agreement with CFT. A similar R-dependence is observed in KCoF₃ in which the CoF₆O_h coordination is maintained in the 0–80 GPa pressure range.

Author(s): Pedro Villanueva-Delgado, Karl W Krämer, Rafael Valiente, Mathijs de Jong, Andries Meijerink

Publication date: 2016

Source: Physical Chemistry Chemical Physics                     Volumen:  18           Issue:            39       Pag.: 27396-27404

DOI: 10.1039/C6CP04347J

Abstract:

Samples of 0.01% and 0.3% Tm³⁺-doped β-NaYF₄ show upconverted UV luminescence at 27660 cm⁻¹ (361 nm) after blue excitation at 21140 cm⁻¹ (473 nm). Contradictory upconversion mechanisms in the literature are reviewed and two of them are investigated in detail. Their agreement with emission and two-color excitation experiments is examined and compared. Decay curves are analyzed using the Inokuti–Hirayama model, an average rate equation model, and a microscopic rate equation model that includes the correct extent of energy transfer. Energy migration is found to be negligible in these samples, and hence the average rate equation model fails to correctly describe the decay curves. The microscopic rate equation model accurately fits the experimental data and reveals the strength and multipolarity of various interactions. This microscopic model is able to determine the most likely upconversion mechanism.

Author(s): Lorena García‐Hevia, Juan C Villegas, Fidel Fernández, Íñigo Casafont, Jesús González, Rafael Valiente, Mónica L Fanarraga

Publication date: 2016/2/1

Source: Advanced healthcare materials          Volumen:  5 Pag.: 1080

DOI: 10.1002/adhm.201500753

Abstract:

Understanding the molecular mechanisms underlying the biosynthetic interactions between particular nanomaterials with specific cells or proteins opens new alternatives in nanomedicine and nanotoxicology. Multiwalled carbon nanotubes (MWCNTs) have long been explored as drug delivery systems and nanomedicines against cancer. There are high expectations for their use in therapy and diagnosis. These filaments can translocate inside cultured cells and intermingle with the protein nanofilaments of the cytoskeleton, interfering with the biomechanics of cell division mimicking the effect of traditional microtubule-binding anti-cancer drugs such as paclitaxel. Here, it is shown how MWCNTs can trigger significant anti-tumoral effects in vivo, in solid malignant melanomas produced by allograft transplantation. Interestingly, the MWCNT anti-tumoral effects are maintained even in solid melanomas generated from paclitaxel-resistant cells. These findings provide great expectation in the development of groundbreaking adjuvant synthetic microtubule-stabilizing chemotherapies to overcome drug resistance in cancer.

Author(s): Lorena García-Hevia, Rafael Valiente, Rosa Martín-Rodríguez, Carlos Renero-Lecuna, Jesús González, Lidia Rodríguez-Fernández, Fernando Aguado, Juan C Villegas, Mónica L Fanarraga

Publication date: 25 Apr 2016

Source: Nanoscale                       Volumen:  8 Issue:            21       Pag.: 10963-10973

DOI: 10.1039/C6NR00391E

Abstract:

Zinc is a crucial element in biology that plays chief catalytic, structural and protein regulatory roles. Excess cytoplasmic zinc is toxic to cells so there are cell-entry and intracellular buffering mechanisms that control intracellular zinc availability. Tubulin and actin are two zinc-scavenging proteins that are essential components of the cellular cytoskeleton implicated in cell division, migration and cellular architecture maintenance. Here we demonstrate how exposure to different ZnO nanostructures, namely ZnO commercial nanoparticles and custom-made ZnO nanowires, produce acute cytotoxic effects in human keratinocytes (HaCat) and epithelial cells (HeLa) triggering a dose-dependent cell retraction and collapse. We show how engulfed ZnO nanoparticles dissolve intracellularly, triggering actin filament bundling and structural changes in microtubules, transforming these highly dynamic 25 nm diameter polymers into rigid macrotubes of tubulin, severely affecting cell proliferation and survival. Our results demonstrate that nano-ZnO causes acute cytoskeletal collapse that triggers necrosis, followed by a late reactive oxygen species (ROS)-dependent apoptotic process.

Author(s): M.A. Sánchez-Alejo, F. Rodríguez, J.A. Barreda-Argüeso, I. Camarillo, C. Flores J., H. Murrieta S., J.M. Hernández A, F. Jaque, E. Camarillo

Source:  Optical Materials Volumen:  60  Pag.: 94

DOI: http://dx.doi.org/10.1016/j.optmat.2016.07.013

Abstract:

In this study, the photoluminescence properties of congruent codoped LiNbO₃:Cr₃.; W⁴⁺, crystals have been systematically investigated by performing photoluminescence studies at room temperature in the 0- 280 kbar pressure range. In particular, we focus on the influence that hydrostatic pressure has on the ²E/ ⁴A₂ (R-lines) transitions of Cr³⁺. It has been observed that the pressure dependence of the spectral position of the R-lines associated with both Cr³⁺ centres and g shows a bilinear behaviour with an abrupt slope change near 210 kbar. This change is related to the existence of a pressure-induced structural phase transition in the LiNbO₃ host. The analysis of experimental results provides the Racah parameters B and C and the crystal field parameter 10Dq and their pressure and volume, through the crystal field theory and equation of state, dependences.

Author(s): Rafael Valiente, Carlos Renero-Lecuna, Fernando Rodríguez, Jesús González

Publication date: 2016/1/31

Source: Journal of Luminescence                    Volumen:  169        Pag.: 410

DOI: http://dx.doi.org/10.1016/j.jlumin.2014.11.043

Abstract:

High-pressure techniques make possible to investigate the changes in the electronic properties induced by modifications of the local or crystal structure of the material without changing the chemical composition. The different sensitivity of excited states to crystal-field strength enables energy tuning of the states, which are eventually responsible for the optical properties. It is possible to induce resonance between levels producing exotic effects like upconversion as well as excited state crossover or high-spin to low-spin transitions. Herein, we present selected examples of high-pressure effect for understanding luminescent phenomena or even inducing new ones.

Author(s): Susana Gómez-Salces, José A Barreda-Argüeso, Rafael Valiente, Fernando Rodríguez

Publication date: 2016/12/31

Source:  Solar Energy Materials and Solar Cells      Volumen:  157   Pag.: 42

DOI: 10.1016/j.solmat.2016.05.010

Abstract:

This work investigates the photoluminescence properties of high transmission glass (HTG) doped with CeO₂ and MnO as well as their capability to transform the solar spectrum by absorption/emission via energy transfer from Ce³⁺ to Mn²⁺ into a more efficient red- enhanced spectrum for traditional Si-based solar cells. We show that Mn³⁺ and Mn²⁺ centers are formed in HTG, their relative concentration depending on the heat treatment and the presence of other dopants like Ce³⁺.Co-doping with CeO₂ aims to stabilize Mn²⁺ …

Author(s): Gloria Almonacid, Rosa Martín-Rodríguez, Carlos Renero-Lecuna, Julio Pellicer-Porres, Saïd Agouram, Rafael Valiente, Jesus Gonzalez, Fernando Rodríguez, Lucie Nataf, Daniel R Gamelin, Alfredo Segura

Publication date: 2016/7/18

Source: Nano letters         Volumen:  16   Issue: 8     Pag.: 5204

DOI: 10.1021/acs.nanolett.6b02230

Abstract:

This paper investigates the electronic structure of wurtzite (W) and rock-salt (RS) Zn1–x Cox O nanoparticles (NPs) by means of optical measurements under pressure (up to 25 GPa), X-ray absorption, and transmission electron microscopy. W-NPs were chemically synthesized at ambient conditions and RS-NPs were obtained by pressure-induced transformation of W- NPs. In contrast to the abrupt phase transition in W–Zn1–xCoxO as thin film or single crystal, occurring sharply at about 9 GPa, spectroscopic signatures of tetrahedral Co2+ …

Author(s): Y. W. Sun,
Ignacio Hernandez, Jesús González, K. Scott, A. Sapelkin, F. Rodríguez, D. J. Dunstan

Publication date: January 2016

Source: The Journal of Physical Chemistry C                       

Abstract:

The study of the G-mode pressure coefficients of carbon nanotubes, reflecting the stiff sp2 bond pressure dependence, is essential to the understanding of their extraordinary mechanical properties as well as fundamental mechanics. However, it is hindered by the availability of carbon nanotubes samples only as bundles or isolated with surfactants. Octadecylamine functionalized carbon nanotubes are mostly of a single diameter and can be stably dispersed in 1, 2-dichloroethane and chloroform without surfactants. Here we perform high pressure Raman spectroscopy on these tubes and obtain their experimental G-mode pressure coefficients for individual tubes and bundles. The + pressure coefficient for bundles is only about half of that for individual tubes in 1, 2-dichloroethane and is about two-thirds in chloroform. The G- pressure coefficient for bundles is about one-third of G+ in 1, 2-dichloroethane and about the same in chloroform. These results for the first time provide unambiguous experimental evidence of the significant effect of bundling on carbon nanotubes’ G-mode pressure coefficients, identifying it as one of the major reasons for the lack of consensus on what the values should be in the literature.

Author(s): Alla Dikhtiarenko, Pedro Villanueva-Delgado, Rafael Valiente, José R García, José Gimeno

Publication date: 2016/2/15

Source: Polymers              Volumen:  8   Issue: 2       Pag.: 48

DOI: http://dx.doi.org/10.3390/polym8020048

Abstract:

A series of 3D oxalate-bridged ruthenium-based coordination polymers with the formula of {[ZII(bpy)₃][MIRu(C₂O₄)₃]}n (ZII = Zn²⁺ (1), Cu²⁺ (3, 4), Ru²⁺ (5, 6), Os²⁺ (7, 8); MI = Li⁺, Na⁺; bpy = 2,2’-bipyridine) and {[ZnII(bpy)₃](H₂O)[LiRu(C₂O₄)₃]}n (2) has been synthesized at room temperature through a self-assembly reaction in aqueous media and characterized by single-crystal and powder X-ray diffraction, elemental analysis, infrared and diffuse reflectance UV–Vis spectroscopy and thermogravimetric analysis. The crystal structures of all compounds comprise chiral 3D honeycomb-like polymeric nets of the srs-type, which possess triangular anionic cages where [ZII(bpy)₃]²⁺ cationic templates are selectively embedded. Structural analysis reveals that the electronic configuration of the cationic guests is affected by electrostatic interaction with the anionic framework. Moreover, the MLCT bands gaps values for 1–8 can be tuned in a rational way by judicious choice of [ZII(bpy)₃]²⁺ guests. The 3D host-guest polymeric architectures can be used as self-supported heterogeneous photocatalysts for the reductive splitting of water, exhibiting photocatalytic activity for the evolution of H₂ under UV light irradiation.

Author(s):  Lorena Garcia-Hevia, Rafael Valiente, Jesus Gonzalez, Jose Luis Fernandez-Luna, Juan C. Villegas and Monica L. Fanarraga

Source: Current Pharmaceutical Design.                   Volumen: 21                       Pag. 1920 – 1929

Abstract: Recent research has opened new alternatives to traditional chemotherapy treatments using nanomaterials as cytotoxic agents. Anti-cancer nanomedicines do not require specific target sites on key proteins or genes to kill cancer cells and have radically different mechanisms to interact with the living matter. Among 1D nanomaterials, multiwalled carbon nanotubes (MWCNTs) have the intrinsic ability to bind tubulin and interfere with microtubule dynamics, mimicking the effect of traditional cytotoxic microtubule-binding agents such as paclitaxel (taxol®). Here, we review the cytotoxic properties of MWCNTs and show a direct pro-apoptotic effect of these nanomaterials in vitro in different cancer cell lines and tumor cells obtained from surgical specimens. Understanding the bio-synthetic relationship between MWCNTs and microtubules could serve to improve these nanomaterials to be used as broad spectrum antineoplastic agents in combination to traditional microtubule-binding treatments, thus avoiding drug resistance mechanisms in cancer cells.

DOI: 10.2174/1381612821666150302144101

Author(s): F. Rodríguez 


Source: International Conference: Study of Matter at Extreme Conditions (SMEC2015)                     Florida International University, Fort Lauderdale (FL, EEUU)         Fecha: 8-15 marzo 2015

Abstract: The optical, electrical and magnetic properties (electronic properties) of transition-metal-ion (TM) compounds strongly depend on the TM ground-state spin. In oxides and halides of Mn³⁺(3d⁴); Co²⁺ (3d⁷), or Fe²⁺ and Co³⁺(3d⁶), the electronic ground state can be relatively close to the spin crossover transition. Hence, subtle structural distortions around the TM due to the Jahn-Teller effect affecting either the low-spin (LS), intermediate-spin (IS) or high-spin (HS) state may favour stabilization of either spin ground state upon compression, eventually yielding metallization. The prediction of which spin state will be stable at given conditions of pressure and temperature is not easy, particularly, in sixfold coordinated TM compounds where the Jahn-Teller effect can play a crucial role. In many cases their structural characterization is hard to accomplish through diffraction techniques due to topological disorder induced by dynamics or spin state mixing and thus adequate structural characterization often deserves using complementary techniques. Here we present a high-pressure spectroscopy study to explore the spin state and associated structural distortions, and the insulating to metal transition in TM halides undergoing spin crossover transitions upon compression. The combination of optical spectroscopy and x-ray diffraction experiments, and ab initio DFT calculations allows us to describe both the structural phase transition sequence, spin crossover phenomena and metallization process in the charge-transfer compound CoCl₂. Besides, we also investigate on the different spin crossover phenomena exhibited by Jahn-Teller systems involving Mn³⁺ and Co³⁺ fluorides. The possible involvement of the strong Jahn-Teller IS state in the spin crossover in CoF₃ will be discussed in connection with high-pressure experiments carried out in the isoelectronic Fe²⁺ in (Mg,Fe)O and its geophysical implications. The reliability of DFT methods to predict spin crossover phenomena will be also discussed.

Author(s): Y.W.Sun, I. Hernández, J. González, I.F. Crowe, M.P. Halsall, K. Scott, K.J. Donovan, A. Sapelkin, D.J. Dunstan, F. Rodríguez

Source: International Conference: Study of Matter at Extreme Conditions (SMEC2015)   Lugar de celebración: Florida International University, Fort Lauderdale (FL, EEUU)       Fecha: 8-15 marzo 2015-

Abstract: Here we present investigations on G-mode pressure dependence of single-wall carbon nanotubes(SWCNT). The aim is to clarify the data dispersion of G-mode pressure shifts reported in theliterature and attributed to distinct factors. Trying to identify the main factors affecting theirpressure dependence, we show that, not only solvents employed as pressure transmitting media, butalso bundling and chirality can significantly influence G-mode pressure coefficients. The study ofG-Mode pressure coefficients, reflecting the stiff sp2 bond pressure dependence, is important tounderstand their extraordinary mechanical properties. However, this information is often hinderedby the availability of CNTs samples as bundles or isolated with surfactants. Here we perform aseries of high pressure Raman spectroscopy experiments on selected SWCNT samples. We obtainexperimental G-Mode pressure coefficients, which are unaffected by surfactants, for individualtubes and bundles, by means of octadecylamine functionalized SWCNTs. The G+ pressurecoefficient for bundles is only about half of that for individual tubes in 1,2-dichloroethane and isabout two-thirds in chloroform. For bundles, the G pressure coefficient is about one-third of G+ in1, 2-dichloroethane and about the same in chloroform. Similarly we find that bundled pristineSWCNTs of (6,5) and (9,11) chiralities having the same diameter show significantly differentpressure coefficients.These results provide experimental evidences of bundling and chirality effects on SWCNT G-modepressure coefficients, claiming them as probable reasons for the dispersion of data found in theliterature

Author(s): M Quintanilla, E Cantelar, F Cussó, J A Barreda-Argüeso, J González, R. Valiente and F Rodríguez

Source:  Optical Materials Express      Volumen:  5 Pag: 1168

DOI:  10.1364/OME.5.001168

Abstract: The cross-relaxation process between the lower excited states of thulium ions has a strong influence on its main infrared emissions, but also in the population of higher excited states that lead to characteristic blue upconversion. This work investigates this process in LiNbO3:Tm³⁺ by means of time-resolved spectroscopy at high pressure. It is demonstrated that through the application of high-pressure it is possible to enhance its probability and to investigate its influence on the photoluminescence spectra and corresponding lifetimes of Tm3^+. The results are analyzed in terms of the effect of high pressure on parameters such as Tm³⁺-Tm³⁺ distance through the Eq. (-)of-state of LiNbO3, refractive index or Tm³⁺– Tm³⁺ energy transfer characteristics (absorption/emission overlap integral), to conclude that the major multipole interaction responsible for crossrelaxation is the quadrupole-quadrupole interaction. This conclusión supports and clarifies previous dynamical models for energy transfer on the basis of spectroscopic studies carried out in LiNbO3:Tm³⁺ as a function of Tm3⁺ concentration.

Author(s):  I Hernández; Y-X. Zheng, M. Motevalli, R.H.C. Tan, W.P. Gillin, P.B. Wyatt 

Source: Congreso: 5^th International Workshop on Photoluminescence in Rare Earths (PRE’14). San Sebastian, 13-16 Mayo 2014

Abstract: Some of our newly produced Yb(III) compounds based on pentachlorotropolonate show the longest emission lifetime and efficiency (180 s, 15%) for non-fluorinated IR-emitting organolanthanides complexes. Interestingly, they are sensitized via low energy lying states in the orange-red region of the spectrum. In this contribution we report the chemical and photophysical characterization and discuss the rules governing the excitation and emission properties. Our work shows that the use of chlorinated ligands allows for an efficient emission by reducing non-radiative quenching and the sensitization via states with a triplet character, which are populated thanks to the enhanced inter-system crossing. These materials may be of interest for devices and applications in telecommunication technologies and biology.

uthor(s):  H. Ye, N. Pathumakanthar, Y.X. Zheng, P.B. Wyatt, W.P. Gillin, I. Hernández

Source: 5^th International Workshop on Photoluminescence in Rare Earths (PRE’14). San Sebastian, 13-16 Mayo 2014

Abstract: Following understanding of the excited state dynamics in long-lived infra-red emitting organolanthanides [1], we have produced relatively efficient infra-red (IR) to visible upconverting polymers based on lanthanide organic complexes [2]. The idea is to exploit the long-lived states of Yb3+ perfluorinated complexes and the cooperative energy transfer to efficient emitters with a wide gap, such as Eu3+ or Tb3+ (which is resonant with the excited Yb pairs’ states). In this paper we report various families of compounds (with different ligands and emitters) showing IR to visible upconversion. We describe some basic design rules derived from the observed behavior and chemical nature, and discuss the advantages, limitations and possibilities of this new kind of compounds

Author(s):  H.O. Ye, Y. Peng, Z. Li, P.B. Wyatt, I. Hernández, W.P. Gillin

Source: 5^th International Workshop on Photoluminescence in Rare Earths (PRE’14). San Sebastian, 13-16 Mayo 2014

Abstract: Organic erbium-doped materials for application of optic amplifier at telecommunication wavelength have been studied for decades. Here we show a potential organic optical gain system which could be operated under low pump power density. The fully-fluorinated organic erbium complex [Er(f-tpip)₃] was mixed with a fully-fluorinated organic zinc complex [Zn(F-BTZ)₂] by using the technique of vacuum-film-deposition. The fabulous sensitization from Zn(F-BTZ)₂ to Er(f-tpip)₃ in a broad visible band (UV ~ 450 nm) has been observed as well as the emission lifetime at 1.54 µm is kept as long as ~ 0.8 ms (the quantum yield is ~ 7%). The sensitization is ~ 10⁴ bigger than intrinsic excitation of erbium. The emission of this co-doped system was going to be saturated when a co-doped film was excited by the low power of a dye laser (~200 mW/cm²). Moreover, this system has been integrated into an organic light emitting diodes (OLED) demonstrating that the energy transfer of sensitization is dominated by triplet states of excited Zn(F-BTZ)₂.

Author(s): Alok M. Srivastava , Carlos Renero-Lecuna , David Santamaría-Pérez , Fernando Rodríguez , Rafael Valiente

Source: Journal of Luminescence                    Vol.:    146                Pag.: 27-32

Abstract: An explanation for the puzzling absence of luminescence from the Pr^{3+ 3}P_J[1/40,1,2] states in C-Ln₂O₃ (cubic; Ln³⁺ =Lu³⁺, Y³⁺, Gd³⁺) family of materials is provided by conducting a study of the emission properties of C-Y₂O₃:Pr³⁺ under applied hydrostatic pressure. Above 7 GPa, electronic transitions from the Pr^{3+ 3}P_J[=0,1,2] states are observed in the emission spectrum of C-Y₂O₃:Pr3+ at room temperature and below. The experimental data reveal that the crystal-field split Pr^{3+ 4}f1⁵d1 configuration is located entirely within the host lattice conduction band and that the promotion of the electron to the Pr^{3+ 4}f₁⁵d₁ state produces a self-trapped exciton-like state with the configuration, [Pr⁴⁺ eCB], where eCB indicates an electron in the host lattice conduction band. Upon excitation, the exciton-like state bypasses the upper emitting ³P_J[=0,1,2] states and directly feeds the lower emitting ¹D₂ state. This explains the absence of optical transitions from the Pr^{3+ 3}P_J[=0,1,2] states in the emission spectrum of C-Y₂O₃:Pr³⁺ At ambient pressure. At high pressures, emission transitions from the Pr^{3+ 3}P_J[=0,1,2] states are observed because of the localization of the Pr^{3+ 4}f₁⁵d₁ state to below the host lattice conduction band edge.

DOI: dx.doi.org/10.1016/j.jlumin.2013.09.028

Author(s): Christopher H. Woodall, Simon K. Brayshaw, Stefanie Schiffers, David R. Allan, Simon Parsons, Rafael Valiente and Paul R. Raithby

Source: CrystEngComm     Vol.:    16                   Pag.: 2087

Abstract: Single crystals of the dithienylethene compounds, 1,2-bis(2-methylbenzothiophen-3-yl)perfluorocyclopentene 1 and 1,2-bis(2,5-dimethylthiophen-3-yl)perfluorocyclopentene 2 undergo pressure-induced single-crystal to single-crystal phase transitions between 4.45–5.35 GPa and 4.15–5.70 GPa, respectively. For 1, there is a smooth reduction in unit-cell volume of ~20% from ambient pressure to 4.45 GPa, followed by a dramatic reduction in volume that coincides with a 7.7% increase in the b axis length. Above the pressure of 5.38 GPa a smooth volume reduction continues. In contrast, for 2, there is a continuous change in unit-cell volume with an observed space group change from C2/c to P21/c, between the pressures of 4.15 and 5.70 GPa. In the crystals of 1 between 4.45 and 5.38 GPa adjacent molecules slide over each other and the dominant stacking interaction changes from a thiophene⋯thiophene interaction at 4.45 GPa to a benzothiophene⋯benzothiophene interaction at 5.38 GPa and, within each molecule, the benzothiophene groups show a significant reorientation at the phase transition. In 2 there is a loss of molecular symmetry, concomitant with the change in space group, at the phase transition with the asymmetric unit changing from containing half a unique molecule to two independent molecules. The molecules show significant reorientations of their ring systems. The nature of the observed transition in 1 was investigated using solid-state computational methods to prove the superior thermodynamic stability of the high-pressure phase to the lower pressure phase at pressures above 5.38 GPa. Solid state UV-Vis spectroscopy of 1, over the pressure range from ambient to 15.4 GPa showed that the compound displayed piezochromism with a significant red shift in the π–π* absorption band and a colour change in the crystal from colourless to red with increasing pressure.

DOI: 10.1039/c3ce41933a

Author(s): Lucie Nataf, José Antonio Barreda-Argüeso, Rafael Valiente, Jesús González, y Fernando Rodríguez

Source: PHYSICAL REVIEW B               Vol.:    89                   Pag.: 115120

Abstract: This work investigates the Mn²⁺ electronic structure and exciton dynamics in one-dimensional (1D) N(CH₃)₄MnCl₃ (TMMC) through time-resolved excitation/emission spectroscopy and absorption measurements in the 0–10 GPa pressure range for different Cu²⁺ doping concentrations. The local and crystal structures have been analyzed by Raman spectroscopy and x-ray absorptionmeasurements at theMnKedge showing that the 1D chain structure is maintained in the whole explored pressure range. We show that both the first Mn²⁺ absorption band, ⁴T₁(G), and its associated emission band experience very large pressure redshifts, which are associated with the crystal anisotropy providing large axial ligand fields at the Mn²⁺ site that increase with pressure. The red emission at 633 nm shows a large pressure variation of 22 nm/GPa (50 meV/GPa) making TMMC a suitable probe for using as a photoluminescence (PL) pressure gauge in the low-pressure regime. The energy-transfer exciton dynamics and trapping at non-PL centers have been explained through changes of the intrachain Mn-Mn exchange interaction and Cu2⁺-trap concentration carried out by applying pressure and doping, respectively. The model demonstrates that an increase of exchange interaction favors both the pumping capability and energy transfer yielding exciton migration. Under these conditions, we show that pressure enhances the PL efficiency of TMMC provided that the Cu²⁺ concentration responsible for the PL quenching is below 0.001 mol %. However, between 0.001% and 0.1%, the PL intensity reduces with pressure, and above 0.1%, the PL is practically quenched even at ambient conditions.

DOI: 10.1103/PhysRevB.89.115120

Author(s):  Y. Peng; H. Ye; Z. Li, M. Montevalli, I. Hernández, W.P. Gillin, P.B. Wyatt

Source: Journal of Physical Chemistry Letters                      Volumen: 5              Pag.: 1560

Abstract: Five new fully fluorinated acylphenoxide ligands, which are aromatic analogues of β-diketonates, provide visible photosensitization of the Er3+  4I13/2 → 4I15/2 emission at 1540 nm (of interest for telecommunications) via the “antenna effect”, as observed in Cs[ErL4] compounds. Depending on the chemical functionalization, the excitation wavelength can be tuned in the 400–650 nm range. Decay times for the solids are in the range of 7–16 μs, proving that the complexes can be of interest for a number of optoelectronic and photonic applications.

Author(s): Carlos Renero-Lecuna, Rosa Martín-Rodríguez, Jesus A. González, Fernando Rodríguez, Gloria Almonacid, Alfredo Segura, Vicente Muñoz-Sanjosé, Daniel R. Gamelin, y Rafael Valiente

Source: Chem. Mater.                   Vol.:    26                   Pag.: 1100−1107

Abstract: This work investigates the electronic structure and photoluminescence properties of Co²⁺-doped ZnO and their pressure and temperature dependences through high-resolution absorption and emission spectroscopy as a function of Co²⁺ concentration and their structural conformations as a single crystal, thin film, nanowire, and nanoparticle. Absorption and emission spectra of diluted ZnO:Co²⁺ (0.01 mol %) can be related to the ⁴T₁(P) → ⁴A₂(F) transition of CoO₄ (Td), contrary to MgAl₂O₄:Co²⁺ and ZnAl₂O₄:Co²⁺ spinels in which the red emission is ascribed to the ²E(G) → ⁴A₂(F) transition. We show that the low-temperature emission band consists of a ⁴T₁(P) zero-phonon line and a phonon-sideband, which is described in terms of the phonon density of states within an intermediate coupling scheme (S = 1.35) involving all ZnO lattice phonons. Increasing pressure to the sample shifts the zerophonon line to higher energy as expected for the ⁴T₁(P) state upon compression. The low-temperature emission quenches above 5 GPa as a consequence of the pressure-induced wurtzite to rocksalt structural phase transition, yielding a change of Co²⁺ coordination from 4-fold Td to 6-fold Oh . We also show that the optical properties of ZnO:Co²⁺ (Td) are similar, independent of the structural conformation of the host and the cobalt concentration. The Co²⁺ enters into regular Zn²⁺ sites in low concentration systems (less than 5% of Co²⁺), although some slight shifts and peak broadening appear as the dimensionality of the sample decreases. These structural effects on the optical spectra are also supported by Raman spectroscopy

DOI: dx.doi.org/10.1021/cm403371n

Author(s):  Y.W. Sun, I. Hernández, A.J. Ghandour, C. Rice, I.F. Crowe, M.P. Halsall, A. Sapelkin, J. Gonzalez, F. Rodriguez and D.J. Dunstan

Source: High Pressure Research.                    Volumen: 4              Pag.191-197

Abstract: We use 488 and 568 nm laser Raman spectroscopy under high pressure to selectively follow evolution of Raman G-mode signals of single-walled carbon nanotubes (SWCNTs) of selected diameters and chiralities ((6, 5) and (6, 4)). The G-mode pressure coefficients of tubes from our previous work are consistent with the thick-wall tube model. Here we report the observation of well-resolved G-minus peaks in the Raman spectrum of SWCNTs in a diamond-anvil cell. The pressure coefficients of these identified tubes in water, however, are unexpected, having the high value of over 9 cm⁻¹ GPa⁻¹ for the G-plus and the G-minus, and surprisingly the shift rates of the same tubes in hexane have clearly lower values. We also report an abrupt increase of G-minus peak width at about 4 GPa superposed on a continuous peak broadening with pressure.

DOI: 10.1080/08957959.2013.878714

Author(s):  H. Ye, Z. Li, Y. Peng, C.-C. Wang, T-Y Li, Y.-X. Zheng, A. Sapelkin, G, Amadopoulos,I. Hernández, P.B. Wyatt, W.P. Gillin

Source: Nature Materials             Volumen: 13                       Pag.: 382

Abstract: Modern telecommunications rely on the transmission and manipulation of optical signals. Optical amplification plays a vital part in this technology, as all components in a real telecommunications system produce some loss. The two main issues with present amplifiers, which rely on erbium ions in a glass matrix, are the difficulty in integration onto a single substrate and the need of high pump power densities to produce gain. Here we show a potential organic optical amplifier material that demonstrates population inversion when pumped from above using low-power visible light. This system is integrated into an organic light-emitting diode demonstrating that electrical pumping can be achieved. This opens the possibility of direct electrically driven optical amplifiers and optical circuits. Our results provide an alternative approach to producing low-cost integrated optics that is compatible with existing silicon photonics and a different route to an effective integrated optics technology.

Haizhou Lu,   William P. Gillin and   Ignacio Hernández

Phys. Chem. Chem. Phys., 2014, Advance Article

DOI: 10.1039/C4CP02028F

In this paper, a series of Er³⁺-doped Y₂O₃ samples are systematically investigated, focusing on the effect of the doping concentration on the emission lifetime and spectrum under both 488 nm and 980 nm excitations. Decay times of the ⁴S_3/2 and ⁴F_9/2 emitting states under 488 nm and 980 nm excitations are found to be different and concentration dependent. We explain these variations in terms of the changes in the up-conversion routes caused by the predominance of energy exchanges that involve the lowest lying excited states.

Author(s): Jose Antonio Barreda-Argüeso, Lucie Nataf, Yamilet Rodríguez-Lazcano, Fernando Aguado, Jesús González, Rafael Valiente, Fernando Rodríguez, Heribert Wilhelm and Andrew P. Jephcoat

Source: Inorg. Chem.                   Vol.:    53                  Pag.: 10708−10715

Abstract: This work reports an X-ray diffraction, X-ray absorption, and Raman spectroscopy study of [(CH₃)4N]₂MnX₄ (X = Cl, Br) under pressure. We show that both compounds share a similar phase diagram with pressure. A P2₁/c monoclinic structure describes precisely the [(CH₃)₄N]₂MnCl₄ crystal in the 0.1−6 GPa range, prior to cristal decomposition and amorphization, while [(CH₃)₄N]₂MnBr₄ can be described by a Pmcn orthorhombic structure in its stability pressure range of 0−3 GPa. These materials are attractive systems for pressure studies since they are readily compressible through the weak interaction between organic/inorganic [(CH₃)₄N⁺/MnX₄²⁻] tetrahedra through hydrogen bonds and contrast with the small compressibility of both tetrahedra. Here we determine the equation-of-state (EOS) of each crystal and compare it with the corresponding local EOS of the MnX₄²⁻ and (CH₃)₄N⁺ tetrahedra, the compressibility of which is an order and 2 orders of magnitude smaller than the crystal compressibility, respectively, in both chloride and bromide. The variations of the Mn−Cl bond distance obtained by extended X-ray absorption fine structure and the frequency of the totally symmetric ν1(A1) Raman mode of MnCl₄²⁻ with pressure in [(CH₃)₄N]₂MnCl₄ allowed us to determine the associated Grüneisen parameter (γloc = 1.15) and hence an accurate local EOS. On the basis of a local compressibility model, we obtained the Grüneisen parameters and corresponding variations of the intramolecular Mn−Br and C−N bond distances of MnBr42− (γloc = 1.45) and (CH₃)₄N⁺ (γloc = 3.0) in [(CH₃)₄N]₂MnBr₄.

DOI: dx.doi.org/10.1021/ic501854g

Author(s): Juan C. Villegas , Laura Álvarez-Montes , Lidia Rodríguez-Fernández , Jesús González , Rafael Valiente, and Mónica L. Fanarraga

Source: Advance Healthcare Materials                        Vol.:    3                     Pag.: 424-432

Abstract: The intranasal drug delivery route provides exciting expectations regarding the application of engineered nanomaterials as nano-medicines or drugdelivery vectors into the brain. Among nanomaterials, multiwalled CNTs (MWCNTs) are some of the best candidates for brain cancer therapy since they are well known to go across cellular barriers and display an intrinsic ability to block cancer cell proliferation triggering apoptosis. This study reveals that microglial cells, the brain macrophages and putative vehicles for MWCNTs into the brain, undergo a dose-dependent cell division arrest and apoptosis when treated with MWCNTs. Moreover, it is shown that MWCNTs severely interfere with both cell migration and phagocytosis in live microglia. These results lead to a re-evaluation of the safety of inhaled airborne CNTs and provide strategic clues of how to biocompatibilize MWCNTs to reduce brain macrophage damage and to develop new nanodrugs.

DOI: 10.1002/adhm.201300178

Author(s):  Fraser J. Kettles, Victoria A. Milway, Floriana Tuna, Rafael Valiente, Lynne H. Thomas, Wolfgang Wernsdorfer, Stefan T. Ochsenbein and Mark Murrie

Source: Inorg. Chem        Volumen: 53                       Pag.: 8970−8978

Abstract: New {TbCu3} and {DyCu3} single-molecule magnets (SMMs) containing a low-symmetry LnIII center (shape measurements relative to a trigonal dodecahedron and biaugmented trigonal prism are 2.2−2.3) surrounded by three CuII metalloligands are reported. SMM behavior is confirmed by frequency-dependent out-of-phase ac susceptibility signals and single-crystal temperature and sweep rate dependent hysteresis loops. The ferromagnetic exchange interactions between the central LnIII ion and the three CuII ions could be accurately measured by inelastic neutron scattering (INS) spectroscopy and modeled effectively. The excitations observed by INS correspond to flipping of CuII spins and appear at energies similar to the thermodynamic barrier for relaxation of the magnetization, ∼15−20 K, and are thus at the origin of the SMM behavior. The magnetic quantum number Mtot of the cluster ground state of {DyCu3} is an integer, whereas it is a half-integer for {TbCu3}, which explains their vastly different quantum tunneling of the magnetization behavior despite similar energy barriers

DOI: dx.doi.org/10.1021/ic500885

Author(s):  S. F. León-Luis, V. Monteseguro, U. R. Rodríguez-Mendoza, M. Rathaiah, V. Venkatramu, A. D. Lozano-Gorrín,a R. Valiente, A. Muñoz and V. Lavín

Source: RSC Adv.              Volumen: 4              Pag.:57691

Abstract: The temperature-dependent green luminescence of Y3Ga5O12 nano-garnets doped with different concentrations of Er3+ ions has been measured from 300 to 850 K and, in more detail, in the biological range from 292 to 335 K. The green emissions were obtained by excitation under 488 nm blue or 800 nm near-infrared laser radiations. Both excitations give rise to bright green luminescence that can be seen by the naked eye, and which can be associated either with Stokes processes, i.e. multiphonon relaxations followed by green spontaneous emission, in the former case or with infrared-to-visible upconversion processes in the latter. The temperature-induced changes in the Er3+ green emissions have been calibrated for both excitations and results point to a strong dependence on the concentration of optically active Er3+ ions. The maximum value of the thermal sensitivity, 64 ! 10″4 K»1 at 547 K, has been obtained for the nano-garnets doped with the lowest concentration of Er3+ ions, which is one of the highest values found in the literature. These results allow to conclude that a relatively low concentration of optically active ions is advisable and the changes induced by temperature on the Green emissions are independent of the laser excitation radiation used, which is necessary to calibrate the temperature of the immediate environment of the Er3+-doped Y3Ga5O12 nano-garnets.

DOI: 10.1039/c4ra11565a

Author(s):  H. Ye, Z. Li, Y. Peng, C.-C. Wang, T-Y Li, Y.-X. Zheng, A. Sapelkin, G, Amadopoulos,I. Hernández, P.B. Wyatt, W.P. Gillin

Source: Nature Materials             Volumen: 13                       Pag.: 382

Abstract: Modern telecommunications rely on the transmission and manipulation of optical signals. Optical amplification plays a vital part in this technology, as all components in a real telecommunications system produce some loss. The two main issues with present amplifiers, which rely on erbium ions in a glass matrix, are the difficulty in integration onto a single substrate and the need of high pump power densities to produce gain. Here we show a potential organic optical amplifier material that demonstrates population inversion when pumped from above using low-power visible light. This system is integrated into an organic light-emitting diode demonstrating that electrical pumping can be achieved. This opens the possibility of direct electrically driven optical amplifiers and optical circuits. Our results provide an alternative approach to producing low-cost integrated optics that is compatible with existing silicon photonics and a different route to an effective integrated optics technology.

Author(s):  P. E. Lizcano-Amorocho, J. A. Barreda-Argüeso, F. Aguado, R. Valiente, J. González and F. Rodríguez

Source: 52nd EHPRG Meeting, Lyon, France, 7-12 September 2014

Abstract: Mixed organic/inorganic crystals as [(CH3)4N]FeCl4 (Ref. 1)are receiving considerable attention due to their simple structure consisting of organic (CH3)4N+ and inorganic FeCl4^– tetrahedra, being precursors of magnetic ionic liquids having the FeCl4 ^– as the most common anion.2 The incorporation of magnetic anions as FeCl4 ^– makes it attractive for modifying their thermodynamic properties by applying external magnetic fields or illumination.3 In spite of its relevance the vibrational and electronic structures of FeCl4 – anions remain yet unexplored in part due to the scarce number of mixed organic/inorganic crystals incorporating FeCl4 ^– anions in the structure, and the strong absorption of these materials in the visible-ultraviolet range, due to lowlying, spin and parity allowed electric-dipole chargetransfer transitions, forming the material band gap. Furthermore, these materials can be easily compressed due to the small bulk modulus (K = 10 GPa) in comparison to the local bulk moduli of both organic and inorganic tetrahedra, being more than an order of magnitude higher. This work reports investigations on the structure, vibrational and electronic structures of [(CH3)4N]FeCl4 as a function of pressure with the aim of determining the compression mechanisms governing the bulk and tetrahedra. The charge-transfer electronic structure of FeCl4 – (Ref. 4) has been investigated in the Fe3+-diluted isomorphous material [(CH3)4N]GaCl4. We report on the existence of this compound as well as on its crystal, vibrational and electronic characterization under high-pressure conditions. The charge-transfer spectrum of this compound via doping control (Figure 1) allowed us to report first investigation on the variation of the four charge-transfer bands with pressure and correlate them with the bond length variation derived from Raman spectroscopy. A full report on the structure of these compounds and their associated properties will be presented at the conference.

Author(s):  J. González, F. Rodríguez, R. Valiente

Source: 52nd EHPRG Meeting, Lyon, France, 7-12 September 2014

Abstract: GO is a water-soluble nanomaterial prepared through extensive chemical attack of graphite crystals to introduce oxygen-containing defects in the graphite stack. In GO, a large fraction (0.5–0.6) of carbon is sp³ hybridized and covalently bonded with oxygen in form of epoxy and hydroxyl groups. The remaining carbon is sp² hybridized and bonded either with neighboring carbon atoms or with oxygen in the form of carboxyl and carbonyl groups, which predominantly decorate the edges of the graphene sheets. GO is therefore a 2D network of sp²– and sp³-bonded atoms, in contrast to an ideal graphene sheet, which consists of 100% sp²-hybridized carbon atoms. This unique atomic and electronic structure of GO, consisting of variable sp²/sp³ fractions, opens up possibilities(channels/ways) for new functionalities. The most notable difference between GO and mechanically exfoliated graphene is the optoelectronic properties arising from the presence of a finite band gap [1]. In this work we have used a solution of GO prepared by GRAnPH Nanotech (Spain). The samples are characterized by high resolution transmission electronic microscopy, photoluminescence and Raman spectroscopy. In the Raman spectrum of graphene oxide the G band is broadened and shifted to 1594 cm^-1 whereas the D band at 1363 cm^-1 becomes the prominent feature in the spectrum indicating the creation of sp³ domains due to the extensive oxidation. The intensity ratio of the D and G bands is a measure of the disorder, as expressed by the sp²/sp³ carbon ratio. In order to investigate their structural stability, unpolarized room temperature Raman spectra excited with 514 nm were studied at high pressures. Typical diamond anvils used in high pressure experiments have been substituted by moissanite (6H-SiC) and sapphire anvils to allow the observation of the D band (around 1360 cm^-1) and the second-order Raman scattering without interference [1]. Up to 15 GPa under paraffin we found a pressure coefficient of 6.2 cm^-1GPa^-1 for the tangential G band and 4.1 cm^-1 GPa^-1 for the D band. The unit-cell volume of graphite oxide (GO) pressurized in the presence of water continuously increases, reaching a maximum at 1.3–1.5 GPa. The unit-cell volume of graphite oxide (GO) pressurized in the presence of water continuously increases, reaching a maximum at 1.3–1.5 GPa [1]. Expansion of the lattice by 28–30% significantly exceeds all previously known examples and is due to gradual pressure induced water insertion into the interlayer space of graphene oxide structure. Up to 4 GPa our Raman measurements, using water as the pressure transmitting medium, show an anomaly in the variation of the frequency of the G band with pressure which we attribute to the insertion of water between the layers of the structure of GO. For pressures between 4 and 22 GPa the compressibility of GO is positive again and the pressure coefficient of the G band is 5 cm^-1GPa^-1. A large number of experimental and theoretical studies have been published on the phase transitions of confined water [2] which are considerably different from those of bulk water.

Author(s): F. Rodríguez

Source: Luleå University of Technology    Luleå(Sweden)                                    June (2014)

Abstract: The optical, electrical and magnetic properties of transition-metal-ion (TM) compounds strongly depend on the ground-state spin. In oxides and halides containing Mn³⁺(3d⁴); Ni³⁺(3d⁷), Co³⁺(3d⁶), or Co²⁺(3d⁷), the electronic ground state is close to the spin crossover transition and subtle structural distortions around the TM due to the Jahn-Teller effect occurring at either the low spin (LS) or high-spin (HS) state may favour stabilization of either a LS or a HS ground state upon compression. The prediction of which spin state will be stable at given conditions of pressure and temperature is not easy, particularly, in sixfold coordinated TM compounds where the Jahn-Teller effect can play a crucial role. In most cases their structural characterization is hard to accomplish through diffraction techniques due to topological disorder induced by dynamics or spin state mixing and thus adequate structural characterization often deserves using complementary techniques. In this talk a high-pressure spectroscopy study to explore the spin state and associated structural distortions in TM ions undergoing spin crossover transitions induced by pressure is presented. In particular, we apply this technique for investigating spin crossover phenomena in Jahn-Teller systems of Mn³⁺, Co³⁺. The possible involvement of the intermediate spin (IS) state in the spin crossover in Co³⁺ will be discussed in connection with high-pressure experiments carried out in the isoelectronic Fe²⁺ in (Mg,Fe)O and its geophysical implications. Also diffraction and spectroscopic techniques under high-pressure conditions in the charge-transfer compound CoCl₂ will be presented to analyze the relationship between spin transition and metallization

Author(s): W. Little, A. Karakoulu, I.Hernandez, F. Rodriguez, J.A. Gonzalez, D. Dunstan and A. Sapelkin

Source: European High Pressure Research Group International Meeting (EHPRG51).- 1-6- Septiembre 2013, Queen Mary, University of London, UK

Author(s): Y. Sun, J.E. Proctor, M. Halsall, I.F. Crowe, A. Sapelkin, Ignacio Hernandez, F. Rodriguez, J.A. Gonzalez y D. Dunstan  

Source: European High Pressure Research Group International Meeting (EHPRG51)         Queen Mary, University of London, UK

Author(s): D. Errandonea, F. Aguado, J. Gonzalez, J.A. Barreda, O. Gomis, D. Santamaria-Perez, B. Garcia-Domene, D. Martinez-Garcia, R. Shukla, S.N. Achary, A.K. Tyagi, and. C. Popescu

DOI: 10.1103/PhysRevB.88.214108

Author(s): Carlos Renero-Lecuna, Rosa Martín-Rodríguez, Jesus A. González, Fernando Rodríguez, Gloria Almonacid, Alfredo Segura, Vicente Muñoz-Sanjosé, Daniel R. Gamelin and Rafael Valiente

Source: American Chemical Society    Volume:  108                       Pagina: 166402

Abstract: This work investigates the electronic structure and photoluminescence properties of Co²⁺-doped ZnO and their pressure and temperature dependences through high-resolution absorption and emission spectroscopy as a function of Co²⁺ concentration and their structural conformations as a single crystal, thin film, nanowire, and nanoparticle. Absorption and emission spectra of diluted ZnO: Co²⁺ (0.01 mol %) can be related to the ⁴T₁(P) → ⁴A₂(F) transition of CoO₄ (Td), contrary to MgAl2O₄:Co²⁺ and ZnAl₂O₄:Co²⁺ spinels in which the red emission is ascribed to the ²E(G) → ⁴A₂(F) transition. We show that the low-temperature emission band consists of a 4T₁(P) zero-phonon line and a phonon-sideband, which is described in terms of the phonon density of states within an intermediate coupling scheme (S = 1.35) involving all ZnO lattice phonons. Increasing pressure to the sample shifts the zero-phonon line to higher energy as expected for the ⁴T₁(P) state upon compression. The low-temperature emission quenches above 5 GPa as a consequence of the pressure-induced wurtzite to rock-salt structural phase transition, yielding a change of Co²⁺ coordination from 4-fold Td to 6-fold Oh . We also show that the optical properties of ZnO:Co²⁺ (Td) are similar, independent of the structural conformation of the host and the cobalt concentration. The Co²⁺ enters into regular Zn²⁺ sites in low concentration systems (less than 5% of Co²⁺), although some slight shifts and peak broadening appear as the dimensionality of the sample decreases. These structural effects on the optical spectra are also supported by Raman spectroscopy.

DOI: 10.1021/cm403371n

Author(s): Juan C. Villegas, Laura Álvarez-Montes, Lidia Rodríguez-Fernández, Jesús González, Rafael Valiente and Mónica L. Fanarraga

Source: Adanced. Healthcare Mater      

Abstract: The intranasal drug delivery route provides exciting expectations regarding the application of engineered nanomaterials as nano-medicines or drugdelivery vectors into the brain. Among nanomaterials, multiwalled CNTs (MWCNTs) are some of the best candidates for brain cancer therapy since they are well known to go across cellular barriers and display an intrinsic ability to block cancer cell proliferation triggering apoptosis. This study reveals that microglial cells, the brain macrophages and putative vehicles for MWCNTs into the brain, undergo a dose-dependent cell division arrest and apoptosis when treated with MWCNTs. Moreover, it is shown that MWCNTs severely interfere with both cell migration and phagocytosis in live microglia. These results lead to a re-evaluation of the safety of inhaled airborne CNTs and provide strategic clues of how to biocompatibilize MWCNTs to reduce brain macrophage damage and to develop new nanodrugs.

DOI: 10.1002/adhm.201300178

Author(s): E. del Corro, J. G. Izquierdo, J. González, M. Taravillo and V. G. Baonza

Source: Journal of Raman Spectroscopy          

Abstract: Raman spectroscopy is used to investigate the three-dimensional stress distribution in 6H-silicon carbide (SiC) specimens subjected to stresses up to 3.7GPa along the c-axis. Specifically, the relative Raman shift of the longitudinal optic phonon of 6H-SiC is used to evaluate the local stress across the bulk crystal. For this purpose, an anvil device with opposed 6H-SiC and sapphire specimens was used. After subjecting the anvils to uniaxial load, several series of two-dimensional Raman maps were registered at different depths in the 6H-SiC anvil. The analysis of the Raman spectra reveals an exponential decay of the stress as a function of the depth. A novel phenomenological Grüneisen-like model is introduced here to account for such observation. On the contrary, the in-plane stress analysis shows a radial Gaussian-like distribution regardless the depth, a distinct behavior that is attributed to the appearance of shear stress components. The suitability of both models and their applicability to other materials are discussed, along with some future directions.

Keywords: silicon carbide; strain distribution; 3D imaging; moissanite anvil; shear and in-plane stresses

DOI: 10.1002/jrs.4252

Author(s): Abel García-Saiz, Pedro Migowski, Oriol Vallcorba, Javier Junquera, Jesús Angel Blanco, Jesús Antonio González, María Teresa Fernández-Díaz, Jordi Rius, Jairton Dupont, Jesús Rodríguez Fernández and Imanol de Pedro

Source: Chemistry a European Journal Communication        

Vol.: 20         Pag.: 72–76

Abstract: A new magnetic ionic liquid (MIL) with 3D antiferromagnetic ordering has been synthetized and characterized. The information obtained from magnetic characterization was supplemented by analysis of DFT calculations and the magneto-structural correlations. The result gives no evidence for direct iron-iron interactions, corroborating that the 3D magnetic ordering in MILs takes place via super-exchange coupling containing two diamagnetic atoms intermediaries.

Keywords: density functional calculations, ionic liquids, magnetic properties, structure elucidation

DOI: 10.1002/chem.201303602

Author(s): Jesús Marquina, Chrystian Power, Jesús González, Jean-Marc Broto

Source: Advances in Materials Physics and Chemistry          

Vol.: 3            Pag.: 178-184

Abstract: The electronic structure, band gap, density of states of the (8,8), (14,0) and (12,3) single-walled carbon nanotubes by the SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) method in the framework den-sity-functional theory (DFT) with the generalized gradients approximation (GGA) were studied. Also, we studied the vibrational properties of the (8,8) and (14,0) nanotubes. Only the calculated relaxed geometry for (12,3) nanotube show significant deviations from the ideal rolled graphene sheet configuration. The electronic transition energies of van Hove singularities were studied and compared with previous results. The calculated band structures, density of states and dis-persion curves for all tubes were in good agreement with theoretical and experimental results.

Keywords: Carbon Nanotubes; Electronic Structure; Dispersion Curve

DOI: 10.4236/ampc.2013.32025

Author(s): Jesús González

Source: NanoSpain2013  (ORAL)  205-206

Abstract: GO is a water-soluble nanomaterial prepared through extensive chemical attack of graphite crystals to introduce oxygen-containing defects in the graphite stack. In GO, a large fraction (0.5–0.6) of carbon is sp3 hybridized and covalently bonded with oxygen in form of epoxy and hydroxyl groups. The remaining carbon is sp2 hybridized and bonded either with neighboring carbon atoms or with oxygen in the form of carboxyl and carbonyl groups, which predominantly decorate the edges of the graphene sheets. GO is therefore a 2D network of sp2 and sp3-bonded  atoms, in contrast to an ideal graphene sheet, which consists of 100% sp2-hybridized carbon atoms. This unique atomic and electronic structure of GO, consisting of variable sp2/sp3 fractions, opens up possibilities for new functionalities. The most notable difference between GO and mechanically exfoliated graphene is the optoelectronic properties arising from the presence of a finite band gap [1]. In this work we have used a solution of GO prepared by GRAnPH Nanotech (Spain). The samples are characterized by high resolution transmission electronic microscopy, photoluminescence and Raman spectroscopy. In the Raman spectrum of graphene oxide the G band is broadened and shifted to 1594 cm-1 whereas the D band at 1363 cm-1 becomes the prominent feature in the spectrum indicating the creation of sp3 domains due to the extensive oxidation (see figure 1). The intensity ratio of the D and G bands is a measure of the disorder, as expressed by the sp2/sp3 carbon ratio. In order to investigate their structural stability unpolarized room temperature Raman spectra excited with 514 nm were studied at high pressures. Typical diamond anvils used in high pressure experiments have been substituted by moissanite (6H-SiC) anvils to allow the observation of the D band (around 1360 cm -1) and the second-order Raman scattering without interference [2] (figure 2). Up to 15 GPa we found a pressure coefficient of 6.2 cm-1 GPa-1 for the tangential G band and 4.1 cm-1 GPa-1 for the D band (figure 3). No pressure-induced structural phase transition was observed in the studied pressure range.

Author(s): J. A. Barreda-Argüeso, S. López-Moreno, M. N. Sanz-Ortiz, F. Aguado, R. Valiente, J. González, F. Rodríguez, A. H. Romero, A. Muñoz, L. Nataf, and F. Baudelet

Source: PHYSICAL REVIEW B                           Vol.:    88                  Pag.:  214108

Abstract: We report a complete structural study of CoF₂ under pressure. Its crystal structure and vibrational and electronic properties have been studied both theoretically and experimentally using first- principles density functional theory (DFT) methods, x-ray diffraction, x-ray absorption at Co K-edge experiments, Raman spectroscopy, and optical absorption in the 0–80 GPa range.We have determined the structural phase-transition sequence in CoF₂ and corresponding transition pressures. The results are similar to other transition-metal difluorides such as FeF₂ but different to ZnF₂ andMgF₂, despite that theCo²⁺ size (ionic radius) is similar to Zn²⁺ andMg^2+.We found that the complete phase-transition sequence is tetragonal rutile (P42/mnm)→CaCl₂ type (orthorhombic Pnnm)→distorted PdF₂ (orthorhombic Pbca)+PdF₂ (cubic Pa3´) in coexistence→fluorite (cubic Fm3´m)→cotunnite (orthorhombic Pnma). It was observed that the structural phase transition to the fluorite at 15 GPa involves a drastic change of coordination from sixfold octahedral to eightfold cubic with important modifications in the vibrational and electronic properties.We show that the stabilization of this high-pressure cubic phase is possible under nonhydrostatic conditions since ideal hydrostaticity would stabilize the distorted-fluorite structure (tetragonal I4/mmm) instead. Although the first rutile→CaCl₂-type second-order phase transition is subtle by Raman spectroscopy, it was possible to define it through the broadening of the Eg Raman mode which is split in the CaCl₂-type phase. First-principles DFT calculations are in fair agreement with the experimental Raman mode frequencies, thus providing an accurate description for all vibrational modes and elastic properties of CoF₂ as a function of pressure.

DOI: 10.1103/PhysRevB.88.214108

Author(s): I. de Pedro, A. García-Saiz, J. González, I. Ruiz de Larramendi, T. Rojo, Carlos A. M. Afonso, Svilen P. Simeonov, J. C. Waerenborgh, Jesús A. Blanco,B. Ramajog and J. Rodríguez Fernández

Source: Physical. Chemistry         Vol.:    15                  Pag.:  12724–12733

Abstract: A novel organic ionic plastic crystal (OIPC) based on a quaternary ammonium cation and a tetrachloroferrate anion has been synthesized with the intention of combining the properties of the ionic plastic crystal and the magnetism originating from the iron incorporated in the anion. The thermal analysis of the obtained OIPC showed a solid–solid phase transition below room temperature and a high melting point above 220 1C, indicating their plastic crystalline behaviour over a wide temperature range, as well as thermal stability up to approximately 200 1C. The magnetization measurements show the presence of three-dimensional antiferromagnetic ordering below 4 K. The results from electrochemical characterization display a solid-state ionic conduction sufficiently high and stable (between 10_2.7 and 10_3.6 S cm_1 from 20 to 180 1C) for electrochemical applications.

DOI: 10.1039/c3cp50749a

Author(s): F. J. Manjón, R. Vilaplana, O. Gomis, E. Pérez-González, D. Santamarıía-Pérez, V. Marín-Borrás, A. Segura, J. González, P. Rodríguez-Hernández, A. Muñoz, C. Drasar, V. Kucek, and V. Muñoz-Sanjose

Source: Physical Status Solidi B             Vol.:    250, 488                   Pag.:  669–676

Abstract: Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃ are narrow bandgap semiconductors with tetradymite crystal structure (R-3m) which have been extensively studied along with their alloys due to their promising operation as thermoelectric materials in the temperature range between 300 and 500 K. Studies on these layered semiconductors have increased tremendously in the last years since they have been recently predicted and demonstrated to behave as 3D topological insulators. In particular, a number of high-pressure studies have been done in the recent years in these materials. In this work we summarize the main results of the high-pressure studies performed in this family of semiconductors to date. In particular, we review recent results that address the main characteristics of the pressure-induced electronic topological transition and structural phase transitions observed in this family of compounds. Future high-pressure studies to be performed on these 3D topological insulators are also commented

Keywords:  Bi₂Se₃, Bi₂Te₃, high pressure, Sb₂Te₃, topological insulators

DOI: 10.1002/pssb.201200672

Author(s): Ana C. Perdigón, Defeng Li, Carmen Pesquera, Fernando González, Belén Ortiz, Fernando Aguado and Carmen Blanco

Source: Journal of Materials Chemistry A                      Vol.:    1                    Pag.:  1213

Abstract: High charge micas are ideal materials to be used as porous solid acids because of their extraordinarily high content of framework aluminium atoms and their thermal stability. However, all efforts to reate porosity in these materials have led to a disordered porous structure, since the full process is compromised by the highly layer attractive forces and hence by the incorporation stage of the porous wall precursors between the clay layers. Thermally stable and ordered mesoporous materials were synthesized for the first time from those swelling brittle micas through a surfactant templating approach. The interlayer space was firstly preexpanded using alkylammonium cations, giving rise to a homogeneous organic–inorganic hybrid structure, which directs the polymerization step of the silica source. A regular porous structure in the supermicropore to the small mesopore range (15–25 °A) together with an exclusive acidity was found after sample calcination. The extraordinary content of framework aluminium is responsible for the large number of acid sites, both in the rare medium and strongly acidic regions. The thermal stability of the new synthesised materials was also tested.

DOI: 10.1039/c2ta00543c

Author(s): J.A. Barreda Argüeso,F. Aguado, F. Rodríguez, S.A.T. Redfern

Source:  AIRAPT’13, Seattle, EEUU, 2013                   Vol.:                           Pag.: 

Abstract: Perovskite crystals are attractive for structural studies at high pressure in a wide multidisciplinary science. Perovskite or distorted-perovskite oxides are relevant as solid state devices as many of them exhibit properties such as colossal magnetoresistance, exotic magnetism, or even high T_C superconductivity. In geoscience, the understanding of high-pressure posperovskite phase is noteworthy. In this sense, it must be noted that NaCoF₃ transforms to the postperovskite phase at a moderate pressure (P = 15 GPa) in comparison to oxides [1]. However KCoF₃ seems to lack this transition as it remains in the perovskite structure up to higher pressure [2]. This work reports a structural study in KCoF₃ as a function of pressure in the 0-60 GPa range. Its large pressure perovskite stability is noteworthy. Therefore, this is a unique system to establish correlations between crystal and electronic structures in a high-symmetry phase, where Co²⁺ ions are located in a perfect octahedral environment. Here we present an x-ray diffraction study in correlation with single-crystal optical absorption spectroscopy. The variation of the crystal-field strength and Racah parameters of Co²⁺ with the crystal volume (and Co-F distance) up to 60 GPa are presented, discussed and compared with available data in other structures involving oxides and chlorides.

Author(s): J.A. Barreda Argüeso, F. Aguado, R. Valiente, J. González, H.D. Hochheimer and F. Rodríguez

Source:  EHPRG, London, UK, 2013                  Conferencia 

Abstract: Copper metagermanate (CuGeO₃) is a prototype of inorganic system exhibiting a spin-Peierls transition at low temperature [1]. At room temperature CuGeO₃ belongs to the orthorhombic (Pbmm) [2-4], the unit cell containing two edge-sharing strongly elongated CuO₆ octahedra as a result of the combined Jahn-Teller effect and crystal anisotropy, with two-long and four-short Cu-O bonds of 2.77 and 1.94 Å, respectively [5,6]. They are coupled to form one-dimensional antiferromagnetic Cu-O chains along the c axis.  The band gap at about 3 eV and related optical and magnetic properties strongly depend on the structure of CuO₆ octahedra and how they are coupled in the network of GeO₄ tetrahedra [4-5]. Despite the importance of GeO₄ modulating the crystal structure of CuGeO₃, the optical and magnetic properties are mainly governed by the edge-linked CuO₆ octahedra [5]. Similarly to CuWO₄ [6], such properties are related to the electronic structure of the strongly distorted CuO₆ octahedra, and the oxygen-mediated Cu-Cu exchange interaction, which is modulated by both the distortion and octahedron tilting [6]. CuGeO₃ exhibits a rich structural phase-transition sequence with pressure [4] which is subtly associated with the packing of CuO₆ and GeO₄ polyhedra upon compression, being strongly dependent on the hydrostaticity of the transmitting media.

This work reports investigation on the electronic and vibrational structure of CuGeO₃ by optical absorption and Raman spectroscopy as a function of pressure and temperature. We aim to correlate variations of the band gap, d-d transitions associated with Cu²⁺ and vibrational mode frequencies with corresponding changes of the crystal structure and particularly of the local CuO₆ distortion and orientation. We conclude that Cu²⁺ d-orbital splitting derived from absorption spectroscopy is consistent with the actual elongated distortion of the CuO₆ octahedron at ambient conditions. Its variation with pressure clearly reveals phase transition phenomena, providing information on the CuO₆ local structure. We show that high-pressure phases (IV-IV’) involve two different distorted CuO₆ octahedra, whereas in phase (V) CuO₆ exhibits a large non-centrosymmetric distortion, as suggested by the increase of oscillator strength of the d-d transition by an order of magnitude. In addition, we study pressure-induced structural transitions together with pressure variations of the GeO₄ modes by Raman spectroscopy.

Author(s): Alok M. Srivastava, Carlos Renero-Lecuna, David Santamaría-Pérez, Fernando Rodríguez, Rafael Valiente

Source: Journal of Luminescence                       Vol.:    146                Pag.:  27-32

Abstract: An explanation for the puzzling absence of luminescence from the Pr^{3+ 3}Pπ=1,2 states in C-Ln₂O₃ family of materials is provided by conducting a study of the emission properties of C-Y₂O₃:Pr³⁺ under applied hydrostatic pressure. Above 7GPa, electronic transitions from the Pr^{3+ 3}Pπ=1,2 states are observed in the emission spectrum of C-Y₂O₃:Pr³⁺ at room temperature and below. The experimental data reveal that the crystal-field split Pr3+ ⁴f₁⁵d₁ configuration is located entirely within the host lattice conduction band and that the promotion of the electron to the Pr3þ 4f15d1 state produces a self-trapped exciton-like state with the configuration, [Pr4þþeCB], where e_CBindicates an electron in the host lattice conduction band. Upon excitation, the exciton-like state by passes the upper emitting ³Pπ=1,2 states and directly feeds the lower emitting 1D2 state. This explains the absence of optical transitions from the Pr^{3+ 3}Pπ=1,2 states intheemissionspectrumofC-Y2O3:Pr3þ at ambient pressure. At high pressures, emission transitions from the Pr^{3+ 3}Pπ=1,2  states are observed because of the localization of the Pr3+ ⁴f₁⁵d₁  state to below the host lattice conduction band edge.

DOI: 10.1016/j.jlumin.2013.09.028

Keywords: Cubic-Y₂O₃ Pr³+, Photoluminescence, High pressure, Self-trapped exciton

Author(s): I. Hernández, Y-X. Zheng, M. Motevalli, R.H.C. Tan, W.P. Gillin and P.B. Wyatt

Source: Chemistry Communication                    Vol.:    49                  Pag.:  1933-1935

Abstract: New Yb(III) complexes based on the pentachlorotropolonate (pctrop) ligand show enhanced infrared emission when excited in the orange organic chromophore. Yb(pctrop)₃(DMF-d₇)₂ presents the highest reported quantum yield for a nonfluorinated infrared-emitting organolanthanide complex.

DOI: 10.1039/C3CC38610D

Author(s): A. J. Ghandour, I. F. Crowe, J. E. Proctor, Y. W. Sun, M. P. Halsall, I. Hernandez, A. Sapelkin, and D. J. Dunstan

Source: Physical Review B                      Vol.:    87                  Pag.:  085416

Abstract: Studies of the mechanical properties of single-walled carbon nanotubes are hindered by their availability only as ensembles of tubes with a range of diameters. However, tunable Raman spectroscopy is capable of identifying individual tubes from such ensembles. Interestingly, both the radial breathing mode and, surprisingly, the G-mode pressure coefficients exhibit strong environmental effects, which are largely independent of the nature of the environment. We show that the G-mode pressure coefficient varies with diameter, consistent with the thick-wall tube model. Reappraisal of literature data for graphene and graphite suggests revision of both the G-mode Grüneisen parameter γ and the shear deformation parameter β toward the value of 1.34.

DOI: 10.1103/PhysRevB.87.085416

Author(s): L. Nuccio, M. Willis, L. Schulz, S. Fratini, F. Messina, M. D’Amico, F. L. Pratt, J. S. Lord, I. McKenzie, M. Loth, B. Purushothaman, J. Anthony, M. Heeney, R. M. Wilson, I. Hernández, M. Cannas, K. Sedlak, T. Kreouzis, W. P. Gillin, C. Bernhard, and A. J. Drew

Source: Physical Review Letters             Vol.:    110                Pag.:  216602

Abstract: Despite the great interest organic spintronics has recently attracted, there is only a partial understanding of the fundamental physics behind electron spin relaxation in organic semiconductors. Mechanisms based on hyperfine interaction have been demonstrated, but the role of the spin-orbit interaction remains elusive. Here, we report muon spin spectroscopy and time-resolved photoluminescence measurements on two series of molecular semiconductors in which the strength of the spin-orbit interaction has been systematically modified with a targeted chemical substitution of different atoms at a particular molecular site. We find that the spin-orbit interaction is a significant source of electron spin relaxation in these materials.

DOI: 10.1103/PhysRevLett.110.216602

Author(s): H. Ye, Y. Peng, Z. Li, C.-C. Wang, Y. Zheng, M. Motevalli , P.B. Wyatt , W.P. Gillin and I. Hernandez

Source:        Journal of Physical Chemistry C                Vol.:    117                Pag.:  23970–23975

Abstract: It is expected that fluorinated organic erbium(III) complexes, of interest for optical applications at λ = 1.5 μm, have improved performance with respect to hydrogenated counterparts. However, the intrinsic radiative properties (including the absorption/emission line strengths) of organic Er³⁺ complexes have not been systematically studied and compared up to date. This has precluded the observation of opto-structural correlations as well as a proper characterization of the infrared f-f transitions and thus a lack of meaningful figures for the optical efficiency of these materials at the 1.5 μm emission. We have performed a complete opto-structural correlation study of the oscillator strengths of the f-f transitions of hydrogenated and fluorinated organic erbium(III) complexes, including a Judd-Ofelt analysis. The Judd-Ofelt analysis on the crystals has allowed the study of the interdependence of the chemical nature, structure, and spectroscopic behavior. We observe clear trends that can help the design and understanding of these important infrared emitters for phosphor and opto-electronic applications.

DOI: 10.1021/jp4093282

Author(s): I. Hernández

Source:  EHPRG 51, London, UK, 2013                        Vol.:                           Pag.: 

Abstract: Organic semiconductors of the Zn(II) benzothiazole-phenoxide family have interesting spectroscopic properties, of interest towards electroluminescence and light harvesting.

Structurally, while the hydrogenated compound shows a dimer structure, fluorinated species in which the phenoxide, the benzothiazole or both groups are correspondingly tetra-fluorinated show centrosymmetric crystal structures as arranged by the stacking interactions of the pi systems. Thus, in this series of materials, depending on the chemical composition different pi-H – pi-F or pi-F – pi-F stackings are achieved.

Interestingly, apart from the purely molecular considerations governing the HOMO and LUMO, pi-stacking plays an important role in stabilizing the molecular states and has an influence in the absorption and emission energy. Thus the emission wavelengths are redshifted in the solid structure with respect to solutions and range from 491 nm (fluorinated in the phenoxide ring) to 530 nm (fluorinated on the benzothiazole ring).

Spectroscopic studies of complexes with different composition (fluorination) show that while the emission maximum experiences a variation of 0.33 eV in the range 0-8 GPa complex fluorinated on the benzothiazole rings the one partially fluorinated on the phenoxide rings experiences Delta(Emax)=0.15 eV. The perfluorinated material shows a value Delta(Emax)=0.22 eV in the same range.

We will discuss the high pressure spectroscopic results and how they can help interpret the low pressure phenomenology.

Author(s): Z. Li, A. Dellali, J. Malik, M. Motevalli, R. M. Nix, T. Olukoya, Y. Peng, H. Ye, W. P. Gillin, I. Hernández and P. B. Wyatt

Source: Inorg. Chemistry               Vol.:    52                  Pag.:  1379-1387

Abstract: Fluorinated analogs of the important dinuclear photo- and electroluminescent material [Zn(BTZ)2]2, where H-BTZ = 2-(2-hydroxyphenyl)benzothiazole, are found to be mononuclear. They easily sublime and show broad, bright visible photoluminescence emission spectra. DFT calculations indicate that HOMO and LUMO energy levels in these materials are substantially lowered by fluorination.

DOI: 10.1021/ic302063u

Author(s): Susana Gómez-Salces, Fernando Aguado, Fernando Rodríguez, Rafael Valiente, Jesús González, Raphael Haumont and Jens Kreisel

Source: PHYSICAL REVIEW B       Volume: 85          Pag: 144109

Abstract: BiFeO3 exhibits a complex phase-transition sequence under pressure associated with changes in octahedron tilts and displacements of Bi3+ and Fe3+ cations. Here, we investigate the local structure of Fe3+ as a function of pressure through absorption crystal-field spectroscopy in the 0–18 GPa range.We focus on the influence of phase transitions on the Fe3+ off-center displacement through the energy (E) and oscillator strength (fd−d) of the 4T1 and 4T2 Fe3+ (3d5) bands observed below the band gap (Egap = 2.49 eV) at 1.39 and 1.92 eV, respectively, at ambient conditions. Pressure induces linear redshift of both 4T1 and 4T2 bands, consistent with the compression of the FeO6 octahedron under pressure. On the other hand, the transition oscillator strength (fd−d = 3 × 10−5), enabled by both the exchange mechanism and the off-center Fe3+ distortion, slightly increases with pressure. The absence of notable anomalies in the variation of E(P) and fd−d (P) through the phase sequence from the ferroelectric rhombohedral R3c phase to the nonpolar orthorhombic Pnma phase suggests a persisting off-center position of the Fe3+. While this local polarity is correlated and expected in the ferroelectric R3c phase, its presence in the high-pressure nonpolar Pnma phase indicates the presence of local polar instabilities.

DOI: 10.1103/PhysRevB.85.144109

Author(s): Lidia Rodriguez-Fernandez, Rafael Valiente, Jesús González, Juan C. Villegas and Monica L. Fanarraga

Source: ACSNANO           Volume:    Pag.: 

Abstract: Microtubules are hollow protein cylinders of 25 nm diameter which are implicated in cytokinetics and proliferation in all eukaryotic cells. Here we demonstrate in vivo how multiwalled carbon nanotubes (MWCNTs) interact with microtubules in human cancer cells (HeLa) blocking mitosis and leading to cell death by apoptosis. Our data suggest that, inside the cells, MWCNTs display microtubule biomimetic properties, assisting and enhancing noncentrosomal microtubule polymerization and stabilization. These features might be useful for developing a revolutionary generation of chemotherapeutic agents based on nanomaterials.

DOI: Published online 10.1021/nn302222m

KEYWORDS: MWCNT. microtubules. cell division . HeLa cell. cancer. apoptosis

Author(s): Fernando Aguado and Fernando Rodríguez, Rafael Valiente, Jean-Paul Itíe and Michael Hanfland

Source: PHYSICAL REVIEW B  RAPID COMUNICATIONS         Volume:  85   Pag.:  100101

Abstract: The interplay between the Jahn-Teller (JT) effect and octahedron tilting in transition-metal perovskites is investigated as a function of pressure. Our focus is on its effects on the exchange and electron-phonon interactions, both having a strong influence on materials properties. We demonstrate that the JT distortion in Cu²⁺ and Mn³⁺ is reduced upon compression and is eventually suppressed at pressures above 20 GPa. X-ray diffraction and x-ray absorption measurements in A₂CuCl₄ layer perovskites (A: Rb, CnH_2n+1NH₃; n = 1–3) show that, although pressure slightly reduces the long Cu-Cl distance in comparison to the Cu-Cu distance in the layer, the JT distortion is stable in the 0–20 GPa range. The difference between lattice (βC0 = 0.14 GPa⁻¹) and local CuCl₆ (β0 = 0.016 GPa−1) compressibilities, together with the high stability of the JT distortion, lead to CuCl6 tilts upon compression. The evolution of the elongated CuCl₆ octahedron in A₂CuCl_4, as well as MnF₆ in CsMnF4 and MnO₆ in LaMnO₃ and DyMnO₃, toward a nearly regular octahedron takes place above 20 GPa, in agreement with experimental results and a model analysis based on the JT energy derived from optical absorption spectroscopy: EJT = 0.25–0.45 eV/Cu2+, EJT = 0.45 eV/Mn³⁺ (CsMnF4), and EJT = 0.25 eV/Mn³⁺ (LaMnO₃). The proposed model clarifies controversial results about pressure-induced JT quenching in Cu²⁺ and Mn³⁺ systems, providing an efficient complementary means to predict pressure behavior in perovskites containing JT transition-metal ions.

DOI:  10.1103/PhysRevB.85.100101

Author(s): Lucie Nataf and Fernando Rodriguez and Rafael Valiente

Source: PHYSICAL REVIEW B  Volume:  86  Pag.:  125123

Abstract: Coordination Chemistry

DOI:  10.1103/PhysRevB.86.125123

This work investigates the electronic structure and photoluminescence (PL) of Co²⁺-doped MgAl₂O₄ and their pressure dependence by time-resolved spectroscopy. The variations of the visible absorption band and its associated emission at 663 nm (τ = 130 ns at ambient conditions) with pressure/temperature can be explained on the basis of a configurational energy model. It provides an interpretation for both the electronic structure and the excited-state phenomena yielding photoluminescence emission and the subsequent quenching. We show that there is an excited-state crossover (ESCO) [4T₁(P) ↔ 2E(G)] at ambient pressure, which is responsible for the evolution of the emission spectrum from a broadband emission between 300 K and 100 K to a narrow-line emission at lower temperatures. Contrary to expectations from the Tanabe-Sugano diagram, instead of enhancing ESCO phenomena, pressure reduces PL and even suppresses it (PL quenching) above 6 GPa. We explain such variations in terms of pressure-induced nonradiative relaxation to lower excited states: 2E(G) → 4T₁(F). The variation of PL intensity and its associated lifetime with pressure supports the proposed interpretation.

Author(s): R. Martın-Rodrıguez, J. Gonzalez, R. Valiente, F. Aguado, D. Santamarıa-Perez and F. Rodrıguez

Source: JOURNAL OF APPLIED PHYSICS Volume: 111   Pag.:  063516

Abstract: CdS nanoparticles prepared by a mechanochemical reaction in a planetary ball mill have been investigated by x-ray diffraction, optical absorption, and Raman scattering under high pressure conditions up to 11 GPa. The zinc-blende (ZB) to rock-salt phase transition is observed around 6 GPa in all experiments, the transition pressure being similar to the one measured in CdS colloidal nanocrystals, and much higher than in bulk (around 3 GPa). The direct optical energy gap in ZB-CdS increases with pressure, and suddenly drops when the pressure is raised above 6 GPa,according to the high-pressure indirect-gap behavior. A linear blue-shift of the CdS Raman spectra is observed upon increasing pressure. Both Raman and x-ray diffraction studies indicate that the phase transition has a large hysteresis, making the ZB phase barely recoverable at ambient conditions. Cell parameters and bulk modulus measured in CdS nanoparticles clearly show that the nanoparticles at ambient conditions are subject to an initial pressure in comparison to CdS bulk.

DOI: dx.doi.org/10.1063/1.3697562

Author(s): Fraser J. Douglas, Donald A. MacLaren, Carlos Renero-Lecuna, Robert D. Peacock, Rafael Valiente and Mark Murrie

Source: The Royal Society of Chemistry           Volume:  14  Pag.:  7110–7114

Abstract: We report a simple synthesis of ultra-thin lanthanide oxide nanowires and ribbons via the autoclave-based decomposition of lanthanide oleates within passivating surfactants. Electron microscopy reveals the formation of linear self assemblies of lanthanide oxide nanoparticles that subsequently recrystallize into high aspect ratio materials via an ‘‘imperfect oriented attachment’’ mechanism.

DOI:  10.1039/c2ce25990g

Author(s): Vemula Venkatramu, Sergio F. León-Luis, Ulises R. Rodríguez-Mendoza, Virginia Monteseguro, Francisco J. Manjón, Antonio D. Lozano-Gorrín, Rafael Valiente, Daniel Navarro-Urrios, C. K. Jayasankar, Alfonso Muñoz and Víctor Lavín

Source: Journal of Materials Chemistry             Volume:  22   Pag.:  13788

Abstract: A novel Y_{3(1_x)}Er_3xGa₅O₁₂ nanocrystalline garnet has been synthesized by a sol–gel technique and a complete structural, morphological, vibrational, and optical characterization has been carried out in order to correlate the local structure of the Er3+ ions with their optical properties. The synthesized nanocrystals are found in a single-phase garnet structure with an average grain size of around 60 nm. The good crystalline quality of the garnet structure is confirmed by FTIR and Raman measurements, since the phonon modes of the nano-garnet are similar to those found in the single crystal garnet. Under blue laser excitation, intense green and red visible and 1.5 mm infrared luminescences are observed, whose relative intensities are very sensitive to the Er3+ concentration. The dynamics of these emissions under pulsed laser excitations are analyzed in the framework of different energy transfer interactions. Intense visible upconverted luminescence can be clearly observed by the naked eye for all synthesized Er³⁺-doped Y₃Ga₅O₁₂ nano-garnets under a cw 790 nm laser excitation. The power dependency and the dynamics of the upconverted luminescence confirm the existence of different two-photon upconversion processes for the green and red emissions that strongly depend on the Er3+ concentration, showing the potential of these nano-garnets as excellent candidates for developing new optical devices.

DOI:  10.1039/c2jm31386c

Author(s): A. Segura, V. Panchal, J. F. Sanchez-Royo, V. Marín-Borras, V. Muñoz-Sanjosé, P. Rodrıguez-Hernandez, A. Muñoz, E. Perez-Gonzalez, F. J. Manjon and J. González

Source: PHYSICAL REVIEW B       Volume:    85,Pag.:  195139

Abstract: This paper reports an experimental and theoretical investigation on the electronic structure of bismuth selenide (Bi2Se3) up to 9 GPa. The optical gap of Bi2Se3 increases from 0.17 eV at ambient pressure to 0.45 eV at 8 GPa. The quenching of the Burstein-Moss effect in degenerate samples and the shift of the free-carrier plasma frequency to lower energies reveal a quick decrease of the bulk three-dimensional (3D) electron concentration under pressure. On increasing pressure the behavior of Hall electron concentration and mobility depends on the sample thickness, consistently with a gradual transition from mainly 3D transport at ambient pressure to mainly two-dimensional (2D) transport at high pressure. Two-carrier transport equations confirm the trapping of high-mobility 3D electrons, an effect that can be related to a shallow-to-deep transformation of donor levels, associated with a change in the ordering of the conduction band minima. The high apparent areal density and low electron mobility of 2D electrons are not compatible with their expected properties in a Dirac cone. Measured transport parameters at high pressure are most probably affected by the presence of holes, either in an accumulation surface layer or as minority carriers in the bulk.

DOI: 10.1103/PhysRevB.85.195139

Author(s): Susana Gómez-Salces, Fernando Aguado, Rafael Valiente, and Fernando Rodríguez

Source: Angewandte Chemie                 Volume:  124   Pag.:  9469 –9472

Abstract: Coordination Chemistry

DOI:  10.1002/ange.201202033

Keywords: coordination chemistry, copper Jahn– Teller distortion, structure elucidation

Author(s): J. Ruiz-Fuertes, A. Segura, F. Rodríguez, D. Errandonea and M. N. Sanz-Ortiz

Source: PHYSICAL REVIEW LETTERS           Volume:  108                 Pagina: 166402

Abstract: High-pressure optical-absorption measurements performed in CuWO₄ up to 20 GPa provide experimental evidence of the persistence of the Jahn-Teller (JT) distortion in the whole pressure range both in the low-pressure triclinic and in the high-pressure monoclinic phase. The electron-lattice couplings associated with the e_g (E* e) and t_2g (T * e) orbitals of Cu²⁺ in CuWO₄ are obtained from correlations between the JT distortion of the CuO6 octahedron and the associated structure of Cu²⁺ d-electronic levels. This distortion and its associated JT energy (EJT) decrease upon compression in both phases. However, both the distortion and associated EJT increase sharply at the phase-transition pressure (PPT ¼ 9:9 GPa), and we estimate that the JT distortion persists for a wide pressure range not being suppressed up to 37 GPa. These results shed light on the transition mechanism of multiferroic CuWO₄, suggesting that the pressure-induced structural phase transition is a way to minimize the distortive effects associated with the toughness of the JT distortion.

DOI 10.1103/PhysRevLett.108.166402

Author(s): Nataf Lucie, Valiente Rafael, González Jesús and Rodriguez Fernando

Source: 6th Asian Conference on High Pressure Research, August 8 – 12, 2012, Beijing, China

Abstract: The (CH₃)₄NMnCl₃ crystal (TMMC) consists of linear chains of face-sharing MnCl₆ ^4- octahedra, which has received considerable attention as an ideal one-dimensional (1D) system of exchangecoupled Mn²⁺ ions. The intrachain Mn-Mn interaction is several orders of magnitude bigger than the interchain Mn-Mn interaction [1], hence those properties related to the exchange-coupled Mn²⁺ ions like 1D magnetism [1,2] or 1D excitation transfer (migration) yielding unusually intense photoluminescence (PL) [3,4] were widely investigated on this model material. In contrast to other 2D and 3D Mn²⁺ systems, the atomic arrangement in TMMC provides a 1D excitonic confinement leading to substantial reduction of exciton capture by non-PL traps. Besides the strong axial anisotropy at the Mn2+ site in TMMC is noteworthy. It is responsible for the large crystal-field strength at Mn²⁺ (10Dq = 0.8 eV) yielding an intense PL band at 630 nm at ambient conditions [4,5] with one of the largest pressure shifts: 23 nm/GPa [6]. This makes TMMC attractive for using as pressure gauge, particularly, in low pressure experiments (below tenths of GPa) requiring fine tuning and precise pressure determination [6]. Here we present an exciton dynamics study in TMMC by time-resolved spectroscopy (emission and excitation), optical absorption and time-dependent PL measurements as a function of pressure in correlation with previous magnetic and structural data [1]. Interestingly, pressure provides an attractive way to modulate the intrachain Mn-Mn exchange interaction making it easier the analysis of exciton dynamics (exciton transfer and capture) governing PL properties. Instead of dealing with PL decay curve I(t) shape analysis, we focus on the intrachain energy transfer processes from the averaged lifetime parameter, τav(P), as a function of pressure. The higher accuracy attained in measuring τav(P) with respect to I(t) allows us a precise determination of the Mn-Mn transfer frequency and its dependence on the exchange interaction (pressure). The obtained results rise the question on whether energy transfer takes place according to a random-walk model or whether there is a driving force reducing the exciton capture time at non-PL impurities. The effects of exchange interaction in the exciton migration and PL efficiency will be analyzed.

Author(s): Jesús Marquina, Enmanuel Flahaut, and Jesús González

Source: P hys. Status Solidi RRL   (Rapid Research Letters) Volume:  1-3

Abstract: In this Letter the electronic properties of double-walled carbon nanotubes (DWCNTs) were studied by transmission measurements as a function of hydrostatic pressure up to 10 GPa. The energies of the optical transitions between the Van Hove singularities decrease with increasing pressure, which can be attributed to pressure-induced hybridization and symmetry-breaking effects. We observed a linear behavior in the pressure-induced shift of the optical transitions. This is in good agreement with previous studies on single-walled carbon nanotubes (SWCNTs). High-resolution TEM image of a bundle of DWCNTs (A), histogram of the number of walls (B), and diameter distribution for CNT samples plotted from 100 HRTEM images (C).

DOI 10.1002/pssr.201206280

Keywords double-walled carbon nanotubes, optical absorption, high-pressure effects, Van Hove singularities

Author(s): C. Renero-Lecuna, R. Martín-Rodríguez, G. Almonacid, R. Valiente, A. Segura, V. Muñoz, J. González, F. Rodríguez

Source: 15th International Conference High Pressure Semiconductor Physics        Fecha/Lugar : Montpellier 25th-27th July ORAL (Francia)

Abstract: During the last 50 years a huge amount of research has been made about ZnO due to its optical and electronic properties as a wide band gap semiconductor. This material shows an ample variety of applications in optoelectronics, bioscience, and sensors or even in cosmetics. This research has increased even more the last decade with the development of the nanoscience, paying more attention to ZnO nanostructures (nanoparticles, thin films, nanowires) doped with transition metals ions.

We report an investigation on the optical properties of Co²⁺-doped ZnO at high pressure by optical absorption and photoluminescence. The absorption spectrum of wurtzite W-Z_n1-xCo_xO shows three main differences with respect to pure W-ZnO: i) the fundamental band-to-band absorption edge is shifted to higher photon energies, ii) a charge-transfer-related broad band appears at energies just below (and overlapping) the band-to-band edge, and iii) well defined absorption bands related to d-d transitions of tetrahedral Co²⁺ are observed. All these features remained at pressures up to 8 GPa where the W to RS phase transition takes place. Beyond this pressure an abrupt change in the visible absorption band intensity for the bulk is observed [1] whereas in nanoparticles, the transition is observed at lower pressures ca. 5 GPa, and at 15 GPa for thin films.

Photoluminescence was measured in all the conformations of W-Zn_1-xCo_xO up to the transition pressure. At room temperature (RT) a broad band emission is observed around 1.8 eV. This emission is related to the ⁴T1(P), ²E(G), ²T1(G) to ground state transition (⁴A2), as low temperature luminescence measurements revealed. This vibronic side band arises due to the electron-phonon coupling associated to the ⁴T1(P).

The W-to-RS phase transition is observed in the three optical absorption features: i) the fundamental edge shifts to much higher photon energies, ii) the charge-transfer band virtually disappears (or overlaps the direct transition of the RS phase), and iii) the d-d Co2+ absorption band around 2 eV decreases its intensity by an order of magnitude and shifts to higher energies (2.5 eV) as a consequence of the modification from a tetrahedral to a octahedral coordination symmetry. We also show that Co2+ luminescence drastically reduces and eventually disappears at the W-RS transition pressure.

Also the high-pressure behaviour of the optical phonons in W-Zn1-xCoxO is studied by Raman spectroscopy. The pressure dependence of the zone-centre phonons (E2, A1, and E1) was measured for the W structure up to the hexagonal-to-cubic phase transition near 8 GPa. Above this pressure some non-active Raman modes were observed due to the distortion and the polycrystalline nature of the sample after two phase transitions. The phase transition is fully reversible for the bulk material at ca. 3 GPa.

Author(s): CARLOS RENERO-LECUNA

Source: XI IKKS 2012 (International Krutyn Summer School) – «Cutting Edge Luminescent Materials: Shifting the Frontiers»              Fecha/Lugar : Septiembre 23-29 2012 ORAL

Abstract: During the last 50 years a huge amount of research has been made about ZnO due to its optical and electronic properties as a wide band gap semiconductor. This material shows an ample variety of applications in the optoelectronic industry, bioscience, and sensors or even in cosmetics. This research has increased even more the last decade with the development of the nanoscience, paying more attention to ZnO nanostructures (nanoparticles, thin films, nanowires) doped with transition metals ions.

We report an investigation on the optical properties of Co²⁺-doped ZnO at high pressure by optical absorption, photo luminescence (PL) and Raman spectroscopy. The absorption spectrum of wurtzite W-Zn_1-xCo_xO shows three main differences with respect to pure ZnO: i) the fundamental band-to-band absorption edge is shifted to higher photon energies, ii) a broad band related to charge-transfer transition appears at energies just below (and overlapping) the band-to-band edge, and iii) well defined absorption bands related to d-d transitions of tetrahedral Co²⁺ are observed. All these features are observed at pressures as high as 15 GPa for thin films [1]. By contrast, in nanoparticles, the transition to the rock-salt (RS) phase is observed at lower pressures ca. 5 GPa, and in bulk is observed around 9 GPa and with an abrupt change in the visible absorption band intensity [2]. PL was measured in all the conformations of W-Zn_1-xCo_xO up to the transition pressure. At room temperature (RT) a broad band emission is observed. This emission is related to the 4T₁(P) to ground state transition (4A₂) and a vibronic side band as low temperature and ambient pressure PL measurements revealed. This vibronic side band is due to the electron-phonon coupling associated to the 4T₁(P). The W-to-RS phase transition is observed in the three optical absorption features: i) the fundamental edge shifts to much higher photon energies, ii) the charge-transfer band virtually disappears (or overlaps the direct transition of the RS phase), and iii) the d-d Co²⁺ absorption band around 2 eV decreases its intensity by an order of magnitude and shifts to higher energies (2.5 eV) as a consequence of the modification from a tetrahedral to a octahedral coordination symmetry. The phase transition also changes the emission spectra of the W-Zn_1-xCo_xO. Upwards the transition pressure, the PL disappears abruptly and no peak is measured. Also the high-pressure behaviour of the optical phonons in W-Zn_1-xCo_xO is studied by Raman spectroscopy. The pressure dependence of the zone-centre phonons (E2, A1, and E1) was measured for the W structure up to the hexagonal-to-cubic phase transition near 8 GPa. Above this pressure some non-active Raman modes were observed. In the pressure downstroke the bulk W-Zn_1-xCo_xO transited from the RS to the W phase at ca. 3 GPa. This behaviour is in contrast to the one observed in W-Zn_1-xCo_xO thin films or nanoparticles, since thin films with Co²⁺ and all nanocrystalline Zn1-xCoxO samples behaves in a different way than the bulk, some of them even remains in the RS phase at ambient pressure after the phase transition. Comparison between Co²⁺ luminescence in its 3 different conformations is measured at ambient pressure. PL at room temperature is observed for the first time in NPs.

Author(s): M. N. Sanz-OrtizA, L. Nataf, J. A. Barreda Argüeso, F. Rodríguez, F. Aguado, R.ValienteC, J. González, F. Baudelet

Abstract: MX₂ binary compounds (M: cation; X: O, F, Cl) show an ample variety of crystal structures related to changes of the cation coordination number. Type structures like α-quartz (SiO₂), rutile (TiO₂, MgF₂, MnF₂, CoF₂), fluorite (CaF₂, SrCl₂), cotunnite (PbCl₂) at ambient conditions are examples of such a behaviour. For a given compound, crystal compression induces structural changes yielding increase of coordination number [1]. So application of high external pressure allows us to move along the phase diagram towards more compact structures as it is illustrated in the figure for some alkaline-earth and transition-metal fluorides. In the latter compound family, such phase transformation may involve significant modifications in the material properties as a consequence of the concomitant changes of the 3d-related electronic structure associated with the change of cation coordination (crystal-field strength) and magnetic interactions (M-M distances and superexchange pathways) [2]. This work reports an x-ray absorption study on the local structure variation around Co²⁺ (EK = 7.723 keV) in CoF2 as a function of pressure in the 0 – 300 kbar. It is worth noting the high pressure critical value for the rutile-fluorite phase transition, Pc =150 kbar, in comparison to isomorphous transition-metal fluorides. EXAFS and XANES analysis will be discussed.

Author(s): C. Renero-Lecuna, R. Martín-Rodríguez, G. Almonacid, A. Segura, R. Valiente, J. González, F. Rodríguez

Source: NanoSpain 2012                                    Fecha/Lugar : Santander 27 feb. 1 marzo (SPAIN)

Abstract: Since the last 50 years a huge amount of research has been made about ZnO due to its optical and electronic properties as a wide band gap semiconductor. This research has increased even more the last decade with the development of the nanoscience, paying more attention to ZnO nanostructures (nanoparticles, thin films, nano wires) doped with transition metals ions such as Co²⁺ or Mn²⁺. These materials show a wide variety of applications in the optoelectronic industry, bioscience, sensors or even in cosmetics.

In this work, thin films (TF) were grown following the pulsed laser deposition (PLD) method over a sapphire substrate. A SEM image shows several round shape Zn1_-xCo_xO of ca. 15 nm and thickness about 100 nm. Colloidal nanoparticles of wurtzite (W) Zn_1-xCo_xO were prepared using hydrolysis and condensation of acetates solution in dimethyl sulfoxide [1]. Some TEM images show spherical nanoparticles with an average diameter of ca. 4 nm. ZnOCo single crystals with different Co²⁺ concentrations have been obtained by physical vapour transport (PVT) for comparing their optical and magnetic properties with those observed in nanostructures.

We report an investigation of their optical properties under high pressure and low temperature by means of optical absorption and photoluminescence. In Zn_1-xCo_xO thin films [2], absorption spectrum of W-Zn_1-xCo_xO shows three main differences with respect to pure W-ZnO: i) the fundamental band-to-band absorption edge is shifted to higher photon energies, ii) a broad band related to charge-transfer transition appears at energies just below (and overlapping) the band-to-band edge, and iii) well defined absorption bands related to d-d transitions of tetrahedral Co²⁺ are observed. All these features are observed at pressures as high as 15 GPa for most concentrations. By contrast, in nanoparticles, the transition to the rock-salt (RS) phase is observed at progressively lower pressures as the Co²⁺ concentration increases [2], and in bulk is observed around 9 GPa and with an abrupt change in the visible absorption band intensity.

The transition to the RS phase is observed in the three absorption features: i) the fundamental edge shifts to much higher photon energies, ii) the charge-transfer band virtually disappears (or overlaps the direct transition of the RS phase), and iii) the d-d Co²⁺ absorption band around 2 eV decreases its intensity by a factor 10 and shifts to higher energies (2.5 eV) as a consequence of the modification from a tetrahedral to a octahedral coordination symmetry.

A comparison between low temperature Co²⁺ photoluminescence at around 1.8 eV, which is assigned to the ⁴T₂ à⁴A₂ transition.

The high-pressure behaviour of optical phonons in W-ZnCoO is studied by Raman spectroscopy at room temperature. The pressure dependence of the zone-centre phonons (E₂, A₁, and E₁) was measured for the W structure up to the hexagonal-to-cubic phase transition near 14 GPa. Above this pressure no active mode was observed. In the pressure down-stroke and independently of the Co²⁺ concentration, all studied nanoparticles remain in the RS phase at ambient pressure. This behaviour is also in contrast to the one observed in ZnCoO thin films, in which only films with Co²⁺ concentrations larger than 25% remain in the RS phase at ambient pressure.

Author(s): Fernando Rodriguez

Source: Advanced Seminar on «Perspectives for Neutron Science in Novel & Extreme conditions»           Fecha/Lugar : 27th – 31st May 2012, Zaragoza (Spain)

Abstract: It is commonly accepted that experimental research under Extreme Conditions is mainly led by scientists having access to large facilities of synchrotron and neutron sources. The reason is probably due to the especially dedicated beamlines operating with beams (flux and energy range) and experimental setups that make them unique to explore the structure of condensed matter systems under the hard requirements imposed by sample environments under extreme conditions. Although beamstations incorporate the newest developments in high pressure, low/high temperature or high magnetic field, there are important novel phenomena in hard and soft condensed matter which are yet difficult to investigate with present experimental facilities due to limitations of sample volume, pressure cells or especial environments. Hence future developments in Extreme Conditions science must consider go beyond present instrumental limitations and at the same time offer an adequate infrastructure for users to do reliable and efficient experiments at large facilities. Furthermore, the possibility of conducting experiments at large facilities using either x-ray or neutron beams jointly with standard spectroscopic techniques (Raman, optical absorption, photoluminescence, etc.) is noteworthy as it provides a systematic in situ sample characterization in order to reproduce given experimental conditions. Furthermore this procedure gives a direct link between experimental results obtained at home laboratories or synchrotron facilities, with high-pressure experiments conducted in neutron facilities. In this way, developments in high volume, high pressure, and high magnetic field can be crucial to unravel a large variety of physical phenomena related to the interplay between spin state, electron-phonon coupling and orbital order governing the optical, magnetic and electrical properties, and that can be modified applying external pressure.

Spin transition phenomena occurring in pure or diluted transition-metal oxides like magnetite Fe2O3 [1], magnesiowüstite (Fe,Mg)O [2], multiferroic BiFeO3 [3,4], delafossite (CuFeO2) [5], low-spin cobaltite (La,Sr)CoO3 [6] or high-spin CoF3 or CsMnF4 drive structural changes with pressure affecting their magnetic and electrical properties: highspin (intermediate spin) to low-spin transitions; charge-transfer processes; insulating-metal transitions, etc. These systems illustrate how investigations using neutron probes under high pressure (30-40 GPa) and high magnetic field (20 T) can be crucial to reveal and eventually understand structural modifications and related phenomena yet unsolved in materials science. This is a contribution from the High Pressure & Spectroscopy Group, University of Cantabria.

Author(s): J. Marquina, CH. Power, J.M. Broto, E. Flahaut, and J. Gonzalez

Source: Revista Mexicana de Física             Volume: 57(1)     Issue:                    Pag: 510-517

Abstract: The temperature dependence of the radial breathing modes (RBMs) and the zone-center tangential optical phonons (G-bands) of doublewalled carbon nanotubes (DWCNTs) has been investigated between 300 and 700 K using Raman scattering. As expected, with increasing temperature, the frequencies of the Raman peaks, including the RBMs and G-bands downshift simultaneously. We show here that the temperature dependence of the RBMs can be fitted by a simple linear dependence and different RBMs have different frequency shifts. We observe a noticeable nonlinearity in the temperature dependence of the G-band associated with the outer semiconducting tube G+ext (s). The deviation from the linear trend is due to the contribution of the third-order anharmonic term in the lattice potential energy with a pure temperature effect. An estimated value of 1.5 for the Grüneisen parameter of the G+ext (s) band was found.

DOI:

Keywords: Raman spectroscopy; double wall carbon nanotubes; high-temperature; anharmonicity; Grüneisen parameter

Author(s): Ch. Power, E. Calderón, J. González y J.C. Chervin

Source: Revista Mexicana de Física             Volume: 57(1)     Issue:                    Pag: 35-39

Abstract: In this work, we study the pressure behavior of the optical absorption spectrum of a single crystal AgGaS₂, taking measurements in the infrared energy range from 0.30 up to 0.70 eV for pressures values P below 4GPa and room temperature T, using a diamond anvil cell in combination with infrared micro spectroscopy technique [1]. With this study, we determine the refraction index n variations in terms of pressure within the stability range of the chalcopyrite structure [2-6] as well as the changes under pressure of both the static (ε₀) and high frequency (ε₀₀) dielectric constants. These results can be compared with the experimental values reported by Boyd et al. [7]

DOI: 10.1103/PhysRevB.81.075120

Keywords: I-III-VI2 semiconductor; infrared; high pressure.

Author(s): Chrystian Power, Jesús González, Edgar Belandria, Jean Claude Chervin y Alain Polian

Source  Scientific Journal from the Experimental Faculty of Sciences  Volume: 19 Nº1  Pag: 41-49

Abstract: We conducted the study of the variation in pressure through the diamond anvil cell (DAC) up to 16GPa, of the acoustic vibrational modes in multigrade motor oil SAE 15W SUPRA SJ 40, which is a commercial lubricant of high antioxidant protection, produced in Venezuela with certificate API-SJ, for gasoline engines LPG or CNG. From Brillouin backscattering spectroscopy we have determined the transverse sound speed, corresponding to different phase transitions and within their domains of stability. In the pressure range close to 1GPa we observe the liquid phase followed by the phase transition to the glass state, the no-discontinuity in the Brillouin shift indicates that the phase transition liquid-glass is a phase transition of second order, for pressures up to 4GPa we observe two phase transitions of first order, which we consider amorphous solid phases.

Key words: diamond anvil cell, Brillouin, sound speed, phase transitions.

Author(s): Jesús Marquina, Enmanuel Flahaut, and Jesús González

Source: P hys. Status Solidi RRL   Volume: 1-3

Abstract: T In this Letter the electronic properties of double-walled carbon nanotubes (DWCNTs) were studied by transmission measurements as a function of hydrostatic pressure up to 10 GPa. The energies of the optical transitions between the Van Hove singularities decrease with increasing pressure, which can be attributed to pressure-induced hybridization and symmetry-breaking effects. We bserved  a linear behavior in the pressure-induced shift of the optical transitions. This is in good agreement with previous studies on single-walled carbon nanotubes (SWCNTs). High-resolution TEM image of a bundle of DWCNTs, histogram of the number of walls, and diameter distribution for CNT samples plotted from 100 HRTEM images.

DOI: 10.1002/pssr.201206280

Keywords: double-walled carbon nanotubes, optical absorption, high-pressure effects, Van Hove singularities

Author(s): J. Ruiz-Fuertes,  D. Errandonea, S. Lopez-Moreno, J. Gonzalez, O. Gomis, R. Vilaplana, F. J. Manjon, A. Muñoz, P. Rodrıguez-Hernandez, A. Friedrich, I. A. Tupitsyna, and L. L. Nagornaya

Source: PHYSICAL REVIEW B       Volume: 83          Pag: 214112

Abstract: Raman scattering measurements and lattice-dynamics calculations were performed on magnesium tungstate (MgWO4) under high pressure up to 41 GPa. Experiments were carried out on a selection of pressure media. The influence of nonhydrostaticity on the structural properties of MgWO4 and isomorphic compounds is examined. Under quasihydrostatic conditions, a phase transition was found at 26 GPa in MgWO4. The high-pressure phase is tentatively assigned to a triclinic structure similar to that of CuWO4. We also report and discuss the Raman symmetries, frequencies, and pressure coefficients in the low- and high-pressure phases. In addition, the Raman frequencies for different wolframites are compared and the variation of the mode frequency with the reduced mass across the family is investigated. Finally, the accuracy of theoretical calculations is systematically discussed for MgWO4, MnWO4, FeWO4, CoWO4, NiWO4, ZnWO4, and CdWO4.

DOI: 10.1103/PhysRevB.83.214112

Author(s): R. Vilaplana, O. Gomis, F. J. Manjon, A. Segura, E. Perez-Gonzalez, P. Rodrıguez-Hernandez, A. Muñoz, J. Gonzalez, V. Marın-Borras, V. Muñoz-Sanjose, C. Drasar  and V. Kucek

Source: PHYSICAL REVIEW B       Volume: 84          Pag: 104112

Abstract: We report an experimental and theoretical lattice dynamics study of bismuth telluride (Bi2Te3) up to 23 GPa together with an experimental and theoretical study of the optical absorption and reflection up to 10 GPa. The indirect bandgap of the low-pressure rhombohedral (R-3m) phase (α-Bi2Te3) was observed to decrease with pressure at a rate of −6meV/GPa. In regard to lattice dynamics, Raman-active modes of α-Bi2Te3 were observed up to 7.4 GPa. The pressure dependence of their frequency and width provides evidence of the presence of an electronic-topological transition around 4.0 GPa. Above 7.4 GPa a phase transition is detected to the C2/m structure. On further increasing pressure two additional phase transitions, attributed to the C2/c and disordered bcc (Im-3m) phases, have been observed near 15.5 and 21.6 GPa in good agreement with the structures recently observedbymeans of x-raydiffractionat highpressures inBi2Te3.After release of pressure the sample reverts back to the original rhombohedral phase after considerable hysteresis. Raman- and IR-mode symmetries, frequencies, and pressure coefficients in the different phases are reported and discussed

DOI: 10.1103/PhysRevB.84.104112

Author(s): Zakariae Amghouz, Santiago García-Granda, José R. García, Abraham Clearfield and Rafael Valiente

Source: American Chemical Society                  Volume:  11   Pag.:  5289–5297

Abstract: A series of novel organic_inorganic hybrids based on trivalent lanthanide (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho) and 1,4-phenylbis(phosphonate) formulated as Ln[O₃P(C₆H₄)PO₃H] has been obtained as single phases under hydrothermal conditions. In the praseodymium compound (Za1), single crystals have been obtained and the crystal structure has been determined. Za1 crystallizes in the monoclinic space group, C2/c, with a = 5.6060(4) Å, b = 20.251(7) Å, c = 8.2740(6) Å, β = 108.52(1)_. All other compounds are isostructural to Za1 as confirmed by Rietveld refinement using X-ray powder diffraction data. Compounds are characterized by thermal analyses (TG-MS and SDTA), elemental analysis, IR spectra, and X-ray thermodiffraction analysis. Their visible photoluminescence properties are also discussed.

DOI:  dx.doi.org/10.1021/cg2008254

Author(s): C. Renero-Lecuna, R. Martín-Rodríguez, R. Valiente, J. González, F. Rodríguez, K. W. Kramer and H. U. Gudel

Source: Chemistry of Materials          Volume: 23          Pag: 3442–3448

Abstract: Site-selective spectroscopy in hexagonal β-NaYF4:Er3+,Yb3+has revealed different environments for Er3+ ions (multisite formation). The low-temperature 4S3/2f4I15/2 Er3+green emission depends on the excitation wavelength associated with the 4F7/2 Er3+ level. We have studied the effect of hydrostatic pressure on the green, red, and blue Er3+ emission upon NIR excitation at ∼980 nm, in order to establish the role played by energy resonance conditions and the multiple Er3+ sites due to the disordered structure for the upconversion (UC) process (energy tuning). The variation of photoluminescence spectra and lifetimes as a function of pressure and temperature reveals that the origin of the high green UC efficiency of the β-NaYF4:Er3+,Yb3+ compound is mainly due to the multisite distribution, and the low phonon energy of the host lattice.

DOI: dx.doi.org/10.1021/cm2004227

KEYWORDS: upconversion, NaYF4: Er3+, Yb3+, high pressure, luminescence, multisites, site selective spectroscopy

Author(s): E. del Corro, M. Taravillo, J. Gonzalez, V.G. Baonza

Source: CARBON Elsavier Volume: 49   Pag.:  973-979

Abstract: Raman spectroscopy experiments on double-wall carbon nanotube and highly oriented pyrolytic graphite (HOPG) samples subjected to non-hydrostatic conditions have been conducted in anvil cells to study the effect of the pressure/stress on the bands assigned to defects. Typical diamond anvils used in high pressure experiments have been substituted by moissanite (6H-SiC) and sapphire (Al2O3) anvils to allow the observation of the D band (around 1350 cm_1) and the second-order Raman scattering without interference. We demonstrate that Raman experiments at high pressure provide unique information to probe the mechanical behaviour of carbon materials (CMs).We also show that this can be also a powerful technique to assign controversial spectral features such as those appearing in the second order region of the spectra of CMs. In HOPG samples we find that the D0/D band intensity ratio is independent of stress. The results indicate that an increase of non-hydrostatic stresses on HOPG generates graphitic domains with sizes around 20–30 nm when the sample is recovered to room conditions.

DOI:10.1016/j.carbon.2010.09.064

Author(s): Susana Gómez, Iñigo Urra , Rafael Valiente , Fernando Rodríguez

Source: Elsevier B.V. Solar Energy Materials & Solar Cells                    Volume:  95   Pag.:  2018–2022

Abstract: This work investigates the formation of photoluminescence (PL) centres in high-transmission glasses (HTG) doped with Cu2O and their capability to transform the solar spectrum by absorption/emission via upconversion, downconversion and Stokes-shifted PL into a more efficient spectrum for photovoltaic applications. Both the green PL Cu+ and the non-PL Cu2+ centres are formed in HTG, although their relative concentration depends on the thermal treatment and the presence of other codopants. Given that the absorption spectrum of Cu+ lies around the HTG band gap, measurement of the absorption coefficient a(l) for these absorption bands is not easy due to corrections between the reflection coefficient and chromatic dispersion. We present a procedure, named two-thickness method, to extract the actual absorption coefficient for the spectrum of each formed centre. In addition it provides the relative Cu+/Cu2+ concentration as well as their absolute values. Analysis of the spectra also provides information on the absorption cross section, transition energy and bandwidth of each band, the knowledge of which is essential to check the suitability of such centres for photovoltaic applications in silicon solar cells

DOI:  10.1016/j.solmat.2010.07.022

Keywords: Glasses, Cu+, Cu2+, Solar converter, Luminescence

Author(s): R Martín-Rodríguez, R Valiente, F Rodríguez and M Bettinelli

Source: IOP PUBLISHING NANOTECHNOLOGY                        Volume:  22   Pag.:  265707                                 Online at stacks.iop.org/Nano/22/265707

Abstract: Since the crystal-field strength at the Cr³⁺ site is very close to the excited-state crossover (ESCO), this work investigates the optical properties of Cr³⁺-doped Gd₃Ga₅O₁₂ (GGG) nanoparticles as a function of temperature and pressure in order to establish the effect of the ESCO on the optical behaviour of nanocrystalline GGG. Luminescence, time-resolved emission and lifetime measurements have been performed on GGG:0.5% Cr³⁺ nanoparticles in the 25–300 K temperature range, as well as under hydrostatic pressure up to 20 GPa. We show how low temperature and high pressure progressively transforms Cr^{3+ 4}T₂ →⁴A₂ broadband emission into a ruby-like ²E →⁴A₂ luminescence. This behaviour together with the lifetime dependence on pressure and temperature are explained on the basis of the spin–orbit interaction between the ⁴T₂ and ²E states of Cr³⁺.

DOI:  10.1088/0957-4484/22/26/265707

Author(s): Russell D. L. Johnstone, David Allan, Alistair Lennie, Elna Pidcock, Rafael Valiente, Fernando Rodriguez, Jesus Gonzalez, John Warren and Simon Parsons

Source  Acta Crystallographica Section B   Volume: B67        226–237

Abstract: Bianthrone [10(10-oxoanthracen-9-ylidene)anthracen-9-one] consists of two tricyclic anthraceneone units connected by a carbon–carbon double bond. Crystals of the form obtained under ambient conditions are yellow and contain folded centrosymmetric conformers in which the central ring of the anthraceneone unit is non-planar. When hydrostatic pressure is applied the crystals assume a red colouration which gradually deepens as pressures increases. The colour change is limited in extent to the surface of the crystals, the bulk remaining yellow. Comparison of high-pressure, single-crystal UV–vis spectra and powder diffraction data demonstrate that the colour change is associated with the formation of a polymorph containing a conformer in which the tricyclic fragments are planar and the molecule is twisted about the central C—C bond. Single-crystal diffraction data collected as a function of pressure up to 6.5 GPa reveal the effect of compression on the yellow form, which consists of layers of molecules which stack along the [010] direction. The structure remains in a compressed form of the ambient-pressure phase when subjected to hydrostatic pressure up to 6.5 GPa, and the most prominent effect of pressure is to push the layers closer together. PIXEL calculations show that considerable strain builds up in the crystal as pressure is increased with a number of intermolecular contacts being pushed into destabilizing regions of their potentials

DOI::10.1107/S0108768111009657

Author(s): Marta N Sanz-Ortiz, Fernando Rodríguez, Jesús Rodríguez and Gerard Demazeau

Source: IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER       Volume:23           Issue:                    Pag: 415501

Online at stacks.iop.org/JPhysCM/23/415501

Abstract: The coordination, the electronic structures and the spin of the ground state of Ni₃C (3d₇) and Co₃C (3d₆) introduced as impurities in LaAlO₃ are investigated through optical spectroscopy and magnetic measurements. The unusual trivalent valence state in both transition-metal ions was stabilised via a sol–gel process followed by high oxygen pressure treatments. We show that the crystal-field strength at the nearly Oh transition-metal site in LaAlO₃ locates Ni₃C and Co₃C near the spin state crossover, yielding a low-spin ground state in both cases. We analyse how the interplay between the Jahn–Teller (JT) effect and the spin state affects the magnetic moment of the ion and its temperature dependence. The optical spectra reveal a JT effect associated with a low-spin ground state in Ni₃C and with a thermally populated high-spin low-lying first excited state in Co₃C. The corresponding JT distortions are derived from structural correlations. We conclude that the JT effect is unable to stabilise the intermediate spin state in Co₃C. A low-spin ground state in thermal equilibrium with a high-spin low-lying first excited state is detected in diluted Co₃C-doped LaAlO₃. These results are compared with those obtained in the parent pure compounds LaNiO₃ and LaCoO_3.

Keywords: Localization effects 72.15.Rn, Hopping transport 72.20.Ee, Weak localization 73.20.Fz, Spin-density waves

Doi:10.1088/0953-8984/23/41/415501

Author(s): A.Rivas-Mendoza. F. Gonzalez-Jiménez, J.M. Broto, H.Rakotob and J. González

Source: REVISTA CUBANA DE FÍSICA  (Agosto 2011)

                Volume:28           Issue:                    Pag: No1

Abstract: El estudio de las propiedades eléctricas del compuesto laminar CuFeTe₂ indica que hay tres regímenes de conducción bien diferenciados dependientes de la temperatura. Por debajo de TSDW ~ 300 K se ha reportado la formación de un estado de Ondas de Densidad de Espín (SDW), en el marco de una transición metal no metal. Por debajo de 100 K (~ TSDW/3) el comportamiento de la resistencia eléctrica como una función de la temperatura y el campo magnético se atribuye a los electrones no condensados (cuasi partículas) en el estado SDW. A bajas temperaturas (1.8 – 20K), baja corriente (< 1 mA) y campo magnético (0 <H <6 Tesla), los efectos de localización débil e interacciones electrónicas aparecen. A temperaturas intermedias (20 < T < 100 K) se observa un comportamiento de conductividad hopping. The study of the electrical properties of the layered compound CuFeTe₂ shows that there are three well differentiated conduction regimes depending on the temperature. Below TSDW ~ 300 K the formation of a Spin Density Wave (SDW) state has been reported, in the frame of a metal to non metal transition. Below 100  K (~ TSDW/3) the behavior of the electrical resistance as a function of temperature and magnetic .eld is attributable to the still present not condensed electrons (quasi particles) in the SDW state. At low temperatures (1.8 – 20K), low current (< 1 mA) and magnetic .eld (0<H <6 Tesla), the effects of weak localization and electronic interactions in two dimensions appear. At intermediate temperatures (20 < T < 100 K) a hopping conductivity behavior is observed.

Keywords: Localization effects 72.15.Rn, Hopping transport 72.20.Ee, Weak localization 73.20.Fz, Spin-density waves

Author(s):  Sanz-Ortiz, M. N.,Rodriguez, FHernandez, I, Valiente, R., Kueck, S.

Source: PHYSICAL REVIEW B        Volume: 81          Issue: 4                 Article Number: 045114

Abstract: This work investigates pressure-induced phase transition (PT) and excited-state-crossover effects on the photoluminescence (PL) properties of LiCaAlF₆:Cr³⁺. We report a PL study by means of time-resolved emission, excitation, and lifetime measurements as a function of pressure. We focus on Cr3+ PL variations around pressure-induced trigonal-to-monoclinic first-order PT in LiCaAlF6 at 7 GPa. Moreover, the structural requirements for changing Cr3+ PL from a broadband emission at 1.59 eV (781 nm) at ambient conditions, to a rubylike narrow-line emission at 1.87 eV (663 nm) are analyzed in the 0-35 GPa range. We show how pressure progressively transforms Cr3+ broadband PL into a rubylike emission that becomes the dominant feature of the room-temperature emission spectrum at 28 GPa. This behavior, together with the pressure dependences of the E-2 and T-4(2) excited-states energy and PL lifetime, are explained on the basis of the electron-phonon coupling associated with the T-4(2) and E-2 states. We demonstrate that both excited states interact through spin-orbit coupling yielding Fano resonance rather than antiresonance as is frequently assumed.

KeyWords Plus: CHROMIUM PHOTOLUMINESCENCE; OPTICAL-PROPERTIES; TRANSITION-METAL; TEMPERATURE-DEPENDENCE; ELECTRONIC-STRUCTURE; SINGLE-CRYSTALS; LASER MATERIALS; CR3+ IONS; SPECTROSCOPY; SPECTRA

DOI: 10.1103/PhysRevB.81.045114

Author(s): Kyle W. Galloway, Stephen A. Moggach, Pascal Parois, Alistair R. Lennie, John E. Warren, Euan K. Brechin, Robert D. Peacock, Rafael Valiente, Jesús González, Fernando Rodríguez,

Simon Parsons and Mark Murrie

Source: The Royal Society of Chemistry

Abstract: High pressure single crystal X-ray diffraction and high pressure single crystal electronic spectroscopic studies of a copper(II) citrate dimer are reported, allowing us to correlate the change in local structure at the Cu(II) centres with the observed piezochromism.

DOI: 10.1039/b000000 [DO NOT ALTER/DELETE THIS TEXT]

Author(s):  Marius Millot ; Sylvie George ; Fariba Hatami ; William T. Masselink ; Jean Leotin ; Jesus González ; Jean-Marc Broto

Source: High Pressure Research   Volume: 29    Issue: 4     Pag.: 488–494

Abstract: We present here high-field magneto-photoluminescence experiments on self-assembled InP/GaP quantum dots under high pressure, at cryogenic temperature. We unveil and discuss a very low pressure coefficient (PC) for the direct exciton and the quenching of the strong luminescence at 1.2 GPa. In fact, the magneto-fingerprints, interpreted in the classic Fock–Darwin framework, give access to the evolution of charge carriers confinement and unravel an increase of the exciton effective mass, involved in the low effective PC.

KeyWords Plus: photoluminescence; semiconductors; quantum dots

DOI: 10.1080/08957950903399675

Author(s):  Marius Millot,  Jean-Marc Broto, Jesus Gonzalez, and Fernando Rodríguez,

Source: PHYSICAL REVIEW    Volume:  81     Pag.:  075120

Abstract: Magneto-optical measurements on ruby under high-pressure conditions provided direct determination of the trigonal crystal field acting at the t_2g orbitals of Cr³⁺ in Al₂O₃ (CrO₆) and its dependence with pressure. The correlation study between the measured trigonal splitting and the trigonal distortion at the Al^3+-substituted site indicates that the trigonal splitting increases with pressure whereas the trigonal distortion slightly reduces. The result is interpreted in terms of an enhancement of the electron-lattice coupling due to trigonal distortion upon reduction in the Al-O bond distance, i.e., the Cr-O bond distance R. The observed variations can be explained on the basis of empirical R dependence of the trigonal crystal field as _Vtr_R−n with n=6. It is shown that this exponent does not change when we consider the pressure variation of the local structure around Cr³⁺ obtained from ab initio calculations. By the way, we also demonstrate that a methanol-ethanol mixture is a good pressure transmitting medium at cryogenic temperatures.

DOI:  10.1103/PhysRevB.81.075120

Author(s):  R. Valiente, M. Millot, F. Rodríguez, J. Gonzálezc, J.-M. Broto, S. George, S. García-Revilla,Y. Romanyuk and M. Pollnau

Source  High Pressure Research   Volume: 29    Issue: 4     Pag.: 748–753

Abstract: In this paper, we present the preliminary results of the combined effect of high pressure (up to 7.5 GPa) and strong external magnetic fields (up to 28.5 T) on the photoluminescence (PL) properties of Er3+– Yb3+ co-doped single-crystal thin films of well-oriented KY(WO4)2 at low temperatures. Measurements were carried out under pulsed magnetic field, exciting the Er3+ ions via upconversion mechanisms by a Ti:sapphire laser with the external magnetic field applied along the crystallographic b axis. The detailed study of the green 4S3/2 → 4I15/2 Er3+ PL after infrared excitation around 1μm as a function of pressure and external magnetic field reveals the potential applicability of this system as sensor of high magnetic fields, high pressure and low temperature, simultaneously.

Keywords: photoluminescence; pressure sensor; intense external magnetic field; non-magnetic diamond anvil cell

DOI: 10.1080/08957950903371716

Author(s): Jesús González, Omar Contreras, Chrystian Power y Edgar Belandria

Source: Scientific Journal from the Experimental Faculty of Sciences   Volume:  18 (4)   Pag.:  257 – 264

Abstract: The motor’s oil multi degree SUPRA SJ SAE 15W 40, is a commercial lubricant of high antioxidant protection, elaborated in Venezuela API-SJ certification, for engines to petrol, GLP or GNV, with a viscosity to 40°C of 110 cSt. This study on the variation of the refractive index and the viscosity of this lubricant was developed with a diamond anvil cell, in room temperature, until pressures of 9,2 GPa. Is observed clearly by means of the variation of the width of line of the luminescence of the ruby (Al2O3:Cr+3) a phase transition of the second order of liquid to glass, in approximately 2,25 GPa. Is determine by means of the interferences in the normal transmission and the adjustment of indexes between the lubricant and the grains of ruby, the index of refraction, its dispersion with the energy of the incidental radiation and its variation with the pressure.

Author(s): M N Sanz-Ortiz, F Rodríguez  and R Valiente

Source: JOURNAL OF PHYSICS: CONDENSED MATTER         Volume:  22   Pag.:  125502                                 Online at stacks.iop.org/JPhysCM/22/125502

Abstract: This work investigates the centre distribution of the Cr³⁺ impurity, the dynamical Jahn–Teller effect in the first ⁴T₂ excited state and the thermal shifts of the absorption and emission peaks in LiCaAlF₆:Cr³⁺ by means of time-resolved emission spectroscopy. The electronic and vibrational fine structure observed in both the absorption and emission spectra at low temperature are assigned according to the vibrational modes of the internal (CrF6)3− complex and the lattice modes. Zero-phonon lines associated with ⁴T₂ →⁴A₂ and ²E →⁴A₂ transitions were detected and assigned on the basis of available high pressure data in LiCaAlF₆:Cr³⁺. We have identified the vibrational coupled modes responsible for the vibrational structure of the low temperature emission spectrum and the reduction of the zero-phonon line (ZPL) splitting caused by the dynamical Jahn–Teller effect in the ⁴T₂ excited state (Huang–Rhys factor, Se = 0.92). In addition, from the temperature variation of the emission intensity I (T ), transition energy E(T ) and bandwidth H(T), we obtained the vibrational modes that are coupled to the emitting state. We have evaluated the two main contributions to the photoluminescence thermal shift through thermal expansion and high pressure measurements: the implicit contribution induced by changes of thermal population and the explicit contribution induced by thermal expansion effects—40% and 60% of the total shift, respectively

DOI:  10.1088/0953-8984/22/12/125502

Author(s): R. Martín-Rodríguez, R. Valiente, F. Rodríguez, F. Piccinelli, A. Speghini and M. Bettinelli

Source: PHYSICAL REVIEW B                          Volume:  82   Pag.:  075117                                 Online at stacks.iop.org/JPhysCM/22/295505

Spectroscopic study of Cu²⁺ and Cu⁺ ions in high-transmission glass. Electronic structure and Cu²⁺/Cu⁺ concentrationsAbstract: A detailed spectroscopic study of the upconversion properties in Mn²⁺-Yb³⁺ codoped LaMgAl₁₁O₁₉ is presented. Pulsed and continuous-wave infrared excitation in the ²F_7/2→²F_5/2 Yb³⁺ absorption peaks induces broad Mn²⁺ green emission at 19450 cm⁻¹, which is assigned to the ⁴T₁→⁶A₁ transition in tetrahedral Mn²⁺ and sharp peaks associated with Yb³⁺-pairs luminescence. Both emissions have very different temporal evolution and can be separated by time-resolved spectroscopy. Among the different concentrations under investigation, the 2%Mn²⁺-5%Yb³⁺ codoped system presents the highest upconversion efficiency. The corresponding emission remains visible to the naked eye up to 650 K. The time dependence of the Mn2+ luminescence upon Yb³⁺ excitation shows distinct behaviors for different doping concentrations. The temporal evolution of the intensity for the diluted system doped with 2%Mn²⁺ and 5%Yb³⁺ together with the pure manganese compound doped with 1%Yb³⁺, as well as the temperature dependence of the upconversion emission intensity and lifetime are relevant to identify the underlying upconversion mechanisms. We show that the main processes responsible for upconversion in this doubly transition-metal rare-earth doped oxide are both ground-state absorption _GSA_/excited-state absorption and GSA/energy-transfer upconversion. An analysis of these processes yielding highly efficient luminescence is discussed on the basis of crystal structure and dopants.

DOI:  10.1103/PhysRevB.82.075117

Author(s): Susana Gomez, Iñigo Urra, Rafael Valiente  and Fernando Rodriguez

Source: JOURNAL OF PHYSICS: CONDENSED MATTER         Volume:  22   Pag.:  295505                                 Online at stacks.iop.org/JPhysCM/22/295505

Abstract: This work investigates the formation of photoluminescence centres in high-transmission glass (HTG) doped with Cu2O and their capability to transform the solar spectrum by absorption/emission via Stokes-shifted PL into a more efficient spectrum for photovoltaic applications. Among the centres formed in HTG, both green photoluminescent Cu+ and the non-photoluminescent Cu2+ centres are created but their relative concentration depends on the thermal treatment and the presence of other co-dopants. The measurement of the absorption coefficient α(λ) nearby the HTG optical gap for Cu+ bands is accomplished by following the two-thickness method. This procedure allows us to obtain the actual absorption coefficient for the spectrum of each formed centre, from which we obtain the relative concentration of Cu+/Cu2+ as well as their absolute values. The analysis of the spectra provides information on the absorption cross sections, transition energies and bandwidths, the knowledge of which is essential to check the suitability of such centres for photovoltaic applications in solar cells.

DOI:  10.1088/0953-8984/22/29/295505

Author(s):  E. del Corro, M. Taravillo, J. González, V.G. Baonza

Source  Elsevier  C A R B ON                         Pag: 6214

Abstract: Raman spectroscopy experiments on double-wall carbon nanotube and highly oriented pyrolytic graphite (HOPG) samples subjected to non-hydrostatic conditions have been conducted in anvil cells to study the effect of the pressure/stress on the bands assigned to defects. Typical diamond anvils used in high pressure experiments have been substituted by moissanite (6H-SiC) and sapphire (Al2O3) anvils to allow the observation of the D band (around 1350 cm-1) and the second-order Raman scattering without interference. We demonstrate that Raman experiments at high pressure provide unique information to probe the mechanical behaviour of carbon materials (CMs). We also show that this can be also a powerful technique to assign controversial spectral features such as those appearing in the second order region of the spectra of CMs. In HOPG samples we find that the D´/D band intensity ratio is independent of stress. The results indicate that an increase of non-hydrostatic stresses on HOPG generates graphitic domains with sizes around 20-30 nm when the sample is recovered to room conditions.

.

DOI: 10.1016/j.carbon.2010.09.064

Author(s):  Edgar Belandria, Marius Millot, Jean-Marc Broto, Emmanuel Flahaut, Fernando Rodriguez, Rafael Valiente, Jesus Gonzalez

Source  Elsevier  C A R B ON         Volume: 48          Pag: 2 5 6 6 –2 5 7 2

Abstract: The preparation of highly anisotropic one-dimensional (1D) structures confined into carbon nanotubes (CNTs) in general is a key objective in nanoscience. In this work, capillary effect was used to fill double wall carbon nanotubes (DWCNTs) with trigonal Tellurium. The samples are characterized by high resolution transmission electronic microscopy and Raman spectroscopy. In order to investigate their structural stability and unravel the differences induced by intershell interactions, unpolarized Raman spectra of radial and tangential modes of DWCNTs filled with 1D nanocrystalline Te excited with 514 nm were studied at room temperature and high pressure. Up to 11 GPa we found a pressure coefficient of 3.7 cm_1 GPa_1 for the internal tube and 7 cm_1 GPa_1 for the external tube. In addition, the tangential band of the external and internal tubes broaden and decrease in amplitude. All findings lead to the conclusion that the outer tube acts as a protection shield for the inner tube (at least up 11 GPa). No pressure-induced structural phase transition was observed in the studied range.

DOI: 10.1016/j.carbon.2010.03.036

Author(s):  D P Rojas, L Fernández Barquín, J Rodríguez Fernández, L Rodríguez Fernández and J González

Source  Nanotechnology     Volume: 21          445702(5pp)

Abstract: Specific heat enhancement in several common nanocrystalline metals is established by comparison with their bulk counterparts. Measurements were carried out in Fe, Cu, Ni and binary alloy LaAl2. The excess specific heat is evidenced as a low temperature peak below 65 K and a high temperature slope above 150 K. The experimental data are in good agreement with a model which considers contributions from the grain boundary and core atoms in the nanoparticles. This model is supported by Raman spectroscopy measurements, showing a softening of the frequency phonon modes associated with a size reduction and increase of the atomic disorder. (Some figures in this article are in colour only in the electronic version)

DOI: 10.1088/0957-4484/21/44/445702

Author(s): J. Ruiz-Fuertes, M. N. Sanz-Ortiz, J. González, F. Rodríguez, A. Segura and D. Errandonea

Source: Journal of Physics: Conference Series           Volume: 215                        Pag: 012048

Abstract: The electronic absorption and Raman spectra of CuWO4 are studied as a function ofpressure in the 0 – 20 GPa range. The below-gap absorption bands at 1.15, 1.38 and 1.56 eV correspond to Cu2+ d-transitions split by the Jahn-Teller distortion of CuO6 (Req = 1.98 Å; Rax =2.39 Å; Qq = 0.47 Å). Pressure induces a strong reduction of the JT distortion up to 10 GPa. Above this pressure we observe, by optical absorption and Raman spectroscopy, a first-order phase transition at 11 GPa with phase coexistence in the 10-12 GPa range, as it is confirmed by Raman spectroscopy. The absorption spectra suggest that two different Cu2+ sites are formed in the high pressure phase, each having rather different CuO6 distortion. The more JT distorted CuO6 centre is stable up to 20 GPa. Rather than JT reduction, pressure induces reorientations of the CuO6 octahedra in the high-pressure phase.

DOI: 10.1088/1742-6596/215/1/012048

Author(s):  Sebastien Nanot, Marius Millot, Bertrand Raquet, Jean-Marc Broto, Arnaud Magrez, Jesus Gonzalez

Source: Physica E Elsevier  Volume:  42     Pag.:  2466–2470

Abstract: We present a gate-dependent Raman spectroscopy and electric transport measurements of an individually connected multiwall carbon nanotube in the field-effect transistor configuration at room temperature. We discuss the origin of the four distinct sharp modes in the G-band feature. The overall G-band is blueshifted by tuning the Fermi level under a back-gate voltage. Each mode experiences different energy shifts symmetrical for n and p doping. Assuming that the four peaks can be tentatively assigned to four different shells of the multiwall carbon nanotube,we propose a simple quantitative analysis which unravels intershell charge transfer within the nanotube.

DOI:  doi:10.1016/j.physe.2010.06.006

Author(s):  Marius Millot, Ramon Tena-Zaera, Vicente Munoz-Sanjose, Jean-Marc Broto and Jesus Gonzalez

Source  APPLIED PHYSICS LETTERS          Volume: 96          152103

Abstract: We report Raman spectroscopy measurements on ZnO crystals grown by the vapor transport method and annealed. Vacuum annealing is found to yield single crystals with ultra low density of defects. We focus on the optical E2 phonon linewidth temperature dependence in the 10–500 K range. The linewidth decrease at low temperature is analyzed and discussed in the light of anharmonic up- and down-conversion processes, unveiling strongly different behaviors

DOI: 10.1063/1.3387843

Author(s): Ch. Power, I. Molina, L. Chacón, J. González, A. Jagui Pérez-Kuroki y J.C. Chervin

Source: Revista Mexicana de Física             Volume: 56(3)     Issue:                    Pag: 217–222

Abstract: In this paper the spectrum of optical normal transmission not polarized of TlGaSe2 is measured as a function of pressure up to 27.6 GPa at room temperature. Our results show that in the range of the visible exist the contributions of two direct gaps of energy, which present a linear dependence with the pressure. The weak transition E_A(è) assigned to the fundamental gap of energy, decreases up to 1.5 GPa with a linear coefficien of -5.31x 10^-2 eVGPa^-1 and the transition E_B(è) shows a coefficien of -9.95 x 10^-2 eVGPa^-1 up to 5.3 GPa (limit of pressure in the visible spectrum). The results in the infrared do not show the presence of the transition EA allowing to see only the behavior of the second transition rightly up to the pressure of metallization 24.6 GPa. In the range of pressure studied from 0.0 to 27.6 GPa, the transition EB shows a not linear behavior with the pressure of quadratic coefficien 1.83 x10^-3 eVGPa^-2.

DOI: 10.1103/PhysRevB.81.075120

Keywords: I-III-VI2 semiconductor; optical properties; infrared; high pressure.

Author(s): R. Martín-Rodríguez, R. Valiente, F. Rodríguez and J. Gonzalez

Source: High Pressure Research     Volume: 29    Issue: 4     Pag.: 482–487

Abstract: Yb3+-doped CdS nanoparticles 5 nmin size and with a pure metastable zinc-blende (ZB) single phase have been prepared in a planetary ball mill.We are in the very weak quantum confinement regime, and therefore the direct optical energy gaps of bulk and nanocrystals are similar. The optical absorption edge of CdS nanocrystals is measured as a function of pressure up to 11 GPa. The direct energy gap in the ZB phase increases non-linearly with pressure and the linear pressure coefficient is about 3.2 × 10−2 eV GPa−1 up to 5.5 GPa. When the pressure is raised above 6GPa, the energy gap drops suddenly by about 0.8 eV and the spectral form of the absorption coefficient is typical of semiconductors with indirect-gap transitions. The solid–solid phase transition from the ZB to the rock-salt phase is observed at pressures far in excess of the bulk phase transition pressure of 3GPa.

KeyWords Plus: nanocrystals; optical properties; phase transitions

DOI: 10.1080/08957950903372615

Author(s): Carmen-Mihaela Tılmaciu, Brigitte Soula, Anne-Marie Galibert, Petar Lukanov, Lucien Datas, Jesús González, Luis Fernádez Barquín, Jesús Rodríguez Fernández, Fernando González-Jiménez, Jose Jorge and Emmanuel Flahaut

Source: The Royal Society of Chemistry            Pag:  6664–6666

Abstract: The synthesis and characterization of superparamagnetic iron(III) oxide nanowires confined within double-walled carbon nanotubes by capillary filling with a melted precursor (iron iodide) followed by thermal  treatment is reported for the first time.

DOI: 10.1039/b909035e

Author(s):  L. Nataf ; F. Rodríguez ; R. Valiente ; J. González

Source: High Pressure Research       Volume: 29    Issue: 4     Pag.: 653–659

Abstract: This work investigates the Mn2+ electronic structure and photoluminescence (PL) in  [(CH3)4N]MnCl3 (TMMC) and their variation with pressure by time-resolved excitation and emission measurements in the 0–15 GPa range.We show that both the crystal-field excitation and the  corresponding PL bands experience large pressure redshifts, which are associated with the anisotropic crystal compressibility providing large axial ligand fields at the Mn2+ site in TMMC. These variations are very large for the emission band, which shifts with pressure at a rate of 21 nm/GPa (50 meV/GPa), making TMMC suitable for using as a pressure gauge.We show that TMMC provides precise pressure determinations in low pressure ranges (MPa–GPa), since it has the capability to detect pressure variations of 10MPa.

KeyWords Plus: Mn2+; TMMC; pressure gauge; sensors; photoluminescence; time-resolved spectroscopy

DOI: 10.1080/08957950903414979

Author(s):  J. González, J. Marquina, F. Rodríguez and R. Valiente

 Source  High Pressure Research   Volume: 29    Issue: 4     Pag.: 594–599

Abstract:  We present here a novel and simple physical method for obtaining ZnO nanocrystals, using the pressure cycle method and the membrane diamond anvil cell (MDAC) up to 13GPa at 500 K. The recovered nanocrystals in the wurtzitic ZnO phase (w-ZnO) were characterized by the high-resolution Raman scattering and X-ray diffraction. A single crystal of w-ZnO was used in this work. The high-pressure cell was a MDAC specifically designed for high-temperature studies. The Raman spectra of the high-quality ZnO single crystal and nanocrystals were compared, and we observed both a softening and an asymmetric broadening of the Raman peaks , which are in good agreement with the effect of relaxation of the q-vector selection rule due to quantum size confinement effects. The experimental results confirm the existence of ZnO nanocrystals in the w-ZnO phase with an average diameter of 17 nm.

Keywords: nanocrystals; Raman scattering; phase transitions; large gap semi-conductors

DOI:  10.1080/08957950903417451

Author(s): Valiente, R, Rodriguez, F, Gonzalez, J., Guedel, H. U., Martin-Rodriguez, R., Nataf, L., Sanz-Ortiz, M. N., Kraemer, K.

Source: CHEMICAL PHYSICS LETTERS     Volume: 481        Issue: 1-3             Pages: 149-151    

Abstract: The study of the f < > d transition on Ce3+-doped Cs2NaLuCl6, through the absorption and emission spectra at high pressure, allows us to conclude that the equilibrium Ce-Cl bond distance in the excited state is shorter than in the ground state. The observed band shifts of -530 +/- 20 cm (1)/GPa and -415 +/- 10 cm (1)/GPa in absorption and emission, respectively, reveal reduction of the Stokes shift and the spin-orbit interaction with pressure. (C) 2009 Elsevier B. V. All rights reserved.

KeyWords Plus: ABSORPTION; SPECTRA; CE3+

DOI: 10.1016/j.cplett.2009.09.059

Author(s): Martin-Rodriguez, R., Valiente, R., Bettinelli, M.

Source: APPLIED PHYSICS LETTERS         Volume: 95          Issue: 9                 Article Number: 091913

Abstract: We present a spectroscopic study of the green upconversion luminescence of Mn2+ upon near infrared Yb3+ excitation in LaMgAl11O19 codoped with manganese and ytterbium. Excitation at 975 nm with a diode laser resonant with the F-2(7/2) –> F-2(5/2) Yb3+ transition induces a broad emission band centered at 514 nm and assigned to the T-4(1) –> (6)A(1) transition of tetrahedral Mn2+. The upconversion luminescence can be seen by the naked eye up to 500 K. This finding is a promising step in order to use the Mn2+ – Yb3+ system as a potential candidate for display phosphors by upconversion. (C) 2009 American Institute of Physics.

KeyWords Plus: COOPERATIVE LUMINESCENCE; DOPED CSMNCL3; SPECTROSCOPY; TRANSITION; MN²⁺

DOI: 10.1063/1.3220059

Author(s): Martin-Rodriguez, R., Valiente, R., Pesquera, C., Gonzalez, F.,Blanco, C., Potin, V., de Lucas, M. C. Marco

Source: JOURNAL OF LUMINESCENCE    Volume: 129    Issue: 9    Pages: 1109-1114

Abstract: Y₂O₃:Er³⁺, Yb³⁺ nanocrystals have been obtained by ball milling and using a combustion synthesis procedure. In both cases the nanocrystals have been successfully coated with SiO₂ following the Stober method. The average size of the as-synthesized nanoparticles has been estimated from X-ray diffraction patterns and transmission electron microscopy images. The dependence of the optical properties of these samples on synthesis procedure or dopant concentration has been investigated. Emission, excitation and lifetime measurements have been carried out. Upconversion luminescence has been detected in all samples and an enhancement of the red to green emission ratio has been observed in all samples after infrared compared to visible excitation. The mechanisms responsible for the upconversion phenomena have been discussed. (C) 2009 Elsevier B.V. All rights reserved.

Author Keywords: Nanoparticles; Upconversion; Optical properties; Energy transfer; Coating

KeyWords Plus: UP-CONVERSION; EMISSION

DOI: 10.1016/j.jlumin.2009.05.012

Author(s): Martin-Rodriguez, R.,Valiente, R., Polizzi, S., Bettinelli, M., Speghini, A.,Piccinelli, F.

Source: JOURNAL OF PHYSICAL CHEMISTRY C    Volume: 113    Issue: 28    Pages: 12195-12200    

Abstract: Gd3Ga5O12 and Y3Al5O12 nanocrystalline powders codoped with Yb³⁺ and Tb³⁺ or Eu³⁺ have been prepared using the sol-gel Pechini method. Nanocrystals obtained by this technique exhibit a cubic phase with the Ia3 (d) over bar space group, Particles show a broad distribution of sizes and shapes, ranging from elongated to quasi-spherical particles of tens of nanometers. Optical properties such as photoluminescence and excitation spectra or lifetime measurements have been studied oil these samples. Green/blue or red visible upconversion luminescence from Tb³⁺ D-5(3), D-5(4) -> F-7(J), or Eu3+ D-5(0) -> F-7(J) transitions, respectively, has been observed upon Yb³⁺ F-2(7/2) -> F-2(5/2) excitation at 975 nm. A detailed investigation on the spectroscopy and excited state dynamics of these systems is extremely important in order to understand the upconversion processes.

KeyWords Plus: COOPERATIVE ENERGY-TRANSFER; YB³⁺ ION; NANOPARTICLES; SPECTROSCOPY; EMISSION

DOI: 10.1021/jp901711g

Author(s): Millot, M., Gilliland, S., Broto, J. M., Gonzalez, J., Leotin, J., Chevy, A., Segura, A

Source: PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS    Volume: 246    Issue: 3    Pages: 532-535    

Abstract: We report here first magneto-photoluminescence investigations under high pressure up to 6 GPa on III-VI layered semiconductor InSe. Both diamagnetism and magnetic field induced gap opening driven by Landau quantization became observable by using a 60 T pulsed magnet. The pressure-induced enhancement of the diamagnetic coefficient is consistent with the increase of the dielectric constant under pressure while the evolution of the linear coefficient is consistent with a slight increase of the electron effective mass up to 4 GPa and a direct-to-indirect conduction-band crossover around that pressure. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

KeyWords Plus: TEMPERATURE; GASE

DOI: 10.1002/pssb.200880542

Author(s): Nataf, L, Rodriguez, F., Valiente, R., Ulanov, V.

Source: JOURNAL OF LUMINESCENCE    Volume: 129    Issue: 12    Special Issue: Sp. Iss. SI    Pages: 1602-1605    Published: DEC 2009  

Abstract: This work investigates the origin of novel visible photoluminescence (PL) bands observed in the spinel MgAl²O⁴ Co²⁺ Besides the well-known fourfold-coordinated Co²⁺⁽T-d) PL at 670 nm [N V Kuleshov, V.P. Mikhailov, VG. Scherbitsky, RV. Prokoshin and K.V Yumashev, J Lumin 55 (1993) 265.], a rich structured PL band at 686 nm was also observed that we associate with uncontrolled impurities of sixfold coordinated Cr³⁺⁽O-h) by time-resolved spectroscopy and lifetime measurements and their variation with temperature. We also show that the lifetime of the Co²⁺⁽T-d) emission at 670 nm varies from tau = 6.7 mu s to 780 ns on passing frorn T = 10 to 290 K. This unexpected behaviour for T-d systems is related to the excited-state crossover (T-4(1)(sic)E-2), making the emission band to transform frorn a narrow-like emission from E-2 at low temperature to a broad structureless band from T-4(1) at room temperature. (c) 2009 Elsevier B.V. All rights reserved.

Author Keywords: Co²⁺; Cr³⁺; MgAl²O⁴; Crystal field; Lifetime; Excited state crossover

KeyWords Plus: SATURABLE ABSORBER; TETRAHEDRAL CO²⁺; SPINEL; IONS; ABSORPTION; CRYSTAL; GLASS

DOI: 10.1016/j.jlumin.2008.12.030

Author(s): Rodriguez-Lazcano, Y., Nataf, L., Rodriguez, F.

Source: PHYSICAL REVIEW B    Volume: 80              Issue: 8                 Article Number: 085115   

Abstract: This work investigates the Mn²⁺ photoluminescence (PL) and electronic structure of [(CH₃)4N]₂MnX₄ (X=Cl,Br)  and their variation with pressure through time-resolved spectroscopy in the 0-15 GPa range. The crystal-field excitation and emission spectra are explained on the basis of the MnX42-(T-d) tetrahedra. Their peaks experience large pressure redshifts, which are associated with the big crystal compressibility and the interaction between organic/inorganic ([(CH₃)4N]₂MnX₄ ^2-) tetrahedra. The variation in the Racah parameters and crystal-field splitting with pressure indicates that the excitation and emission redshifts of Mn²⁺ are mainly governed by the increase in the Mn-X bond covalency (70%) rather than the increase in the crystal-field splitting (30%). Above 6 GPa, pressure induces structural modifications, which are related to aggregation of the MnX₄^2- tetrahedra with change in Mn²⁺ coordination from fourfold MnX₄^2-(T-d) to sixfold MnX₆^4-(O-h). This process involves a drastic change in the PL behavior yielding a simultaneous two-color PL emission: green emission at 520 nm and a red emission around 650 nm. Both emissions experience noticeable redshifts with pressure producing a marked piezo-PL effect. The large pressure range of phase coexistence makes these materials attractive for multiband PL, the wavelengths of which can be tuned through pressure and eventually stabilized at ambient conditions.

Author Keywords: angular momentum theory; band structure; bonds (chemical); compressibility; crystal field interactions; high-pressure effects; organic compounds; photoluminescence; piezo-optical effects; red shift; time resolved spectra

KeyWords Plus: ONE-DIMENSIONAL LATTICE; HYDROSTATIC-PRESSURE; SINGLE-CRYSTAL; MANGANESE(II) COMPOUNDS; OPTICAL-PROPERTIES; EMISSION DYNAMICS; PHASE-TRANSITIONS; (CH3)4NMNCL3 TMMC; ROOM-TEMPERATURE; DOPED CRYSTALS

DOI: 10.1103/PhysRevB.80.085115

Author(s):Rodriguez-Lazcano, Y., Nataf, Lucie, Rodriguez, Fernando

Source: HIGH PRESSURE RESEARCH    Volume: 29    Issue: 2    Pages: 230-234

Abstract: This work investigates the Mn2+ electronic structure and photoluminescence (PL) in [(CH₃)₄N]₂MnCl₄ and [(CH₃)₄N]₂MnBr₄ and their variation with pressure by time-resolved excitation and emission spectra in the 0-10GPa range. We show that both the crystal-field excitation and corresponding PL bands experience large-pressure redshifts (about 5nm/GPa), which are associated with the high crystal compressibility and the interaction between organic/inorganic ([CH₃]₄N-/MnX[image omitted]) tetrahedra. The energy variation as a function of pressure is explained in terms of the isolated Td MnX[image omitted] in the respective crystals through crystal-field models. One salient feature is the reduction of the PL Stokes shift with pressure in both crystals as-1.0 and-1.7nm/GPa for chloride and bromide, respectively.

Author Keywords: Mn2+Td; Oh; crystal field; Stokes shift; time-resolved spectroscopy

KeyWords Plus: PHOTOLUMINESCENCE; COMPLEXES

DOI: 10.1080/08957950802564692

Author(s): Rodriguez-Lazcano, Y., Nataf, L., Rodriguez, F

Source: JOURNAL OF LUMINESCENCE    Volume: 129    Issue: 12    Special Issue: Sp. Iss. SI    Pages: 2000-2003

Abstract: This work Investigates the Mn²⁺-photoluminescence (PL) of [(CH₃)₄N-2](2)MnX₄ (X Cl, Br) under high pressure (0-15 GPa). We show that pressure induces structural modifications leading to changes of coordination geometry around Mn2+ from fourfold-coordinated MnX₄² (T-d) to sixfold MnX₆⁴ (O-h) along with we observe enormous redshifts of the green emission at 520 nm as well as a new red emission about 700 nm above 6 GPa. The Mn²⁺-PL behaviour with pressure in these highly efficient materials is investigated by time-resolved spectroscopy (emission and excitation) Under high pressure Furthermore, the two-colour PL emissions induced by pressure experience noticeable redshifts (6 nm/GPa) producing a marked piezo-PL effect. The large pressure hysteresis observed in downstroke makes these materials attractive as a new type for multiband emission, which can be tuned through pressure and eventually stabilised at ambient conditions. (C) 2009 Elsevier B.V. All rights reserved

Author Keywords: Mn²⁺ T-d/O-h; High pressure; Time-resolved spectroscopy; Two-colour luminescence; Organic/inorganic compounds

KeyWords Plus: ONE-DIMENSIONAL LATTICE; MANGANESE(II) COMPOUNDS; EMISSION DYNAMICS; SINGLE-CRYSTAL; DOPED CRYSTALS; TRIBOLUMINESCENCE; PHOTOLUMINESCENCE; LUMINESCENCE; TEMPERATURE; TRANSITIONS

DOI: 10.1016/j.jlumin.2009.04.077

Author(s): Sanz-Ortiz, Marta N., Rodriguez, Fernando

Source: JOURNAL OF CHEMICAL PHYSICS    Volume: 131    Issue: 12  Article Number: 124512 

Abstract: This work investigates the influence of electron-phonon coupling associated with E circle times e and T circle times e Jahn-Teller (JT) effect in different transition-metal (TM) ions on de-excitation phenomena through nonradiative multiphonon relaxation, i.e., photoluminescence (PL) quenching. We developed a configurational curve model which is able to predict from the absorption spectrum whether a given JT-TM ion is PL or quenched. The prediction is made on the basis of an adapted Dexter-Klick-Russell parameter for JT systems, defined in terms of spectroscopic parameters through Lambda(JT) =alpha Delta(abs)(e)/E-abs, where Delta(abs)(e) refers to the splitting of the parent octahedral E-g states by the JT distortion in E circle times e (alpha = 3/4) or T circle times e (alpha = 1/4), and E-abs is the energy of the first absorption band involving electronic transition between E-g and T-2g. We show that PL in any JT-TM ion occurs whenever Lambda(JT)<0.1 or is quenched if Lambda(JT)>0.2. This result is noteworthy since it allows us to establish structural requirements for the JT-TM ion and the host crystal to be PL. Although PL properties of materials containing TM ions depend on a variety of structural factors such as the electronic configuration, the site symmetry, and the crystal field produced by neighboring atoms, the present model achieves this goal through a simple spectroscopic parameter: Lambda(JT). In this work we correlated the PL properties of different sixfold-coordinated JT systems such as Ti³⁺, Cu2+, Mn³⁺, Cr²⁺, Fe²⁺, Co³⁺, and Ni³⁺ in halides and oxides with Lambda(JT) obtained from their respective absorption spectra. From this analysis we conclude that depending on the nature of the JT coupling and its strength, PL is either strongly favored or quenched in T circle times e while it is mostly quenched in E circle times e systems due to the larger JT distortion. (C) 2009 American Institute of Physics.

KeyWords Plus: ANGULAR OVERLAP MODEL; CHROMIUM PHOTOLUMINESCENCE; OPTICAL-PROPERTIES; LUMINESCENCE PROPERTIES; TEMPERATURE-DEPENDENCE; HYDROSTATIC-PRESSURE; CHARGE-TRANSFER; SINGLE-CRYSTAL; SPECTROSCOPY; STATES

DOI: 10.1063/1.3223459

Author(s): Aguado, Fernando, Rodríguez, Fernando, Redfern, Simon A. T.

Source: HIGH PRESSURE RESEARCH    Volume: 29    Issue: 4    Pages: 525-529    Published: 2009  

Abstract: The aim of this work is to study the structural evolution and stability of ACoF3 (A: K, Rb)

undistorted perovskites under high-pressure conditions within the hydrostatic regime (below 10GPa). Attention is paid to their distortion propensity, extending the pressure range for this purpose. The role of A cations on the crystal compressibility, which can shed light on the mechanism governing the high-pressure evolution of oxides, has been explored. The results are compared with previous reports on the ABF3 family and point out the relevance of d-electron bonding in the crystal compressibility.

Author Keywords: perovskite; ACoF3; X-ray diffraction; structural distortions; polyhedron compressibility

KeyWords Plus: HIGH-PRESSURE BEHAVIOR; PHASE-TRANSITION; DIFFRACTION; MGSIO3

DOI: 10.1080/08957950903468041

Author(s): Aguado, Fernando, Rodriguez, Fernando, Hirai, Shigeto, Walsh, Joanna N., Lennie, Alistair, Redfern, Simon A. T.

Source: HIGH PRESSURE RESEARCH    Volume: 28    Issue: 4    Pages: 539-544 

Abstract: The structural stability of cubic KMF₃ (M: Mg, Zn, Co, Ni) perovskites has been studied by powder X-ray diffraction under pressure. Neither superlattice reflections nor peak splitting associated to a phase transition were detected in the 0-10GPa pressure range. Furthermore, KMgF₃ showed no structural changes up to 50GPa. The results were compared with previous reports on isostructural KMnF₃. Tolerance factor and site parameters ratio have been analysed as high-pressure stability indexes for this family of perovskites.

Author Keywords: perovskite; halides; high pressure; structural stability

KeyWords Plus: NEUTRON POWDER DIFFRACTION; CRYSTAL-STRUCTURE; KCUF₃; KMGF₃; KMNF₃

DOI: 10.1080/08957950802576464

Author(s): Garcia-Revilla, S., Rodriguez, F., Valiente, R.

Abstract: Eu²⁺-doped CsCdBr₃ single crystals are studied by polarized variable- temperature optical absorption and luminescence spectroscopy for different excitation wavelengths. Whereas the low-energy absorption band is assigned to f -> d transitions within Eu²⁺ electronic configuration, the high-energy absorption bands are assigned to Eu-trapped exciton due to the proximity of the high-energy d levels to the conduction band. (c) 2007 Elsevier B.V. All rights reserved.

Author Keywords: divalent lanthanides; Eu2+; CsCdBr₃; exciton

KeyWords Plus: RARE-EARTH IONS; INORGANIC-COMPOUNDS; EU²⁺

DOI: 10.1016/j.jlumin.2007.12.034

Author(s): Garcia-Revilla, S., Valiente, R., Romanyuk, Y. E., Pollnau, M.

Source: JOURNAL OF LUMINESCENCE    Volume: 128    Issue: 5-6    Pages: 934-936    Published: MAY-JUN 2008  

Abstract: Crystalline Er3+ and Yb3+ singly and doubly doped KY(WO₄)(2) thin films were grown by low-temperature liquid-phase epitaxy. Absorption, luminescence, excitation and temporal evolution measurements were carried out for both Er3+ and Yb3+ transitions from 10 K to room temperature. Green Er3+ upconversion luminescence was observed after Yb3+ and Er3+ excitation. The mechanisms responsible for the upconversion phenomena detected in each case were identified. (c) 2007 Elsevier B.V. All rights reserved.

Author Keywords: upconversion; KY(WO₄)(₂); Er³⁺; Yb³⁺; thin films

KeyWords Plus: LASER

DOI: 10.1016/j.jlumin.2007.12.025

Author(s): Gonzalez, J., Power, C, Belandria, E., Jorge, J., Gonzalez-Jimenez, F., Millot, M., Nanot, S., Broto, J. M., Flahaut, E.

Source: HIGH PRESSURE RESEARCH    Volume: 28    Issue: 4    Pages: 577-582   

Abstract: The preparation of highly anisotropic one-dimensional (1D) structures confined into carbon nanotubes (CNTs) in general is a key objective in CNTs research. In this work, the capillary effect was used to fill double wall carbon nanotubes with iron. The samples are characterized by Mossbauer and Raman spectroscopy, transmission electron microscopy, scanning area electron diffraction, and magnetization. In order to investigate their structural stability and compare it with that of single wall carbon nanotubes (SWNTs), elucidating the differences induced by the inner-outer tube interaction, unpolarized Raman spectra of tangential modes of double wall carbon nanotubes (DWNTs) filled with 1D nanocrystallin -Fe excited with 514 nm were studied at room temperature and elevated pressure. Up to 16GPa we find a pressure coefficient for the internal tube of 4.3cm-1GPa-1 and for the external tube of 5.5cm-1GPa-1. In addition, the tangential band of the external and internal tubes broadens and decreases in amplitude. All findings lead to the conclusion that the outer tube acts as a protection shield for the inner tubes (at least up 16GPa). Structural phase transitions were not observed in this range of pressure.

Author Keywords: carbon nanotubes; Raman scattering; high pressure

KeyWords Plus: NANOWIRES; MOSSBAUER; ELECTRON

DOI: 10.1080/08957950802485757

Author(s): Hernandez, I., Rodriguez, F, Hochheimer, H. D.

Source: JOURNAL OF LUMINESCENCE    Volume: 128    Issue: 5-6    Pages: 1007-1009   

Abstract: A novel two-color photoluminescence (PL) is found in MnF₂ at room temperature (RT) under high pressure. In contrast to the low-temperature PL, the observation of room-temperature PL is unusual in transition-metal concentrated materials like MnF₂ since the de-excitation process at RT is fully governed by energy transfer to non-radiative centers. We show that room-temperature MnF₂ emissions originate in the pressure-induced cotunnite phase. Both the nine-fold Mn²⁺ coordination and the Mn-F-Mn exchange pathway inhibit exciton migration among Mn²⁺ favoring two-band PL at RT. The electronic structure and the excited-state dynamics are investigated by time-resolved emission and excitation spectroscopies under pressure. The two PL bands at 2.34 and 1.87eV above 15 GPa are assigned to Mn2+ emissions arising from two distinct Mn²⁺ centers formed in the MnF2 high-pressure phase. The microscopic origin of the two-color PL is analyzed in terms of exciton dynamics in the MnF₂ cotunnite structure. (C) 2007 Elsevier B.V. All rights reserved.

Author Keywords: MnF2; high-pressure; time-resolved spectroscopy; energy transfer

DOI: 10.1016/j.jlumin.2007.10.030

Author(s): Hernandez, Ignacio, Rodriguez, Fernando, Tressaud, Alain

Source: INORGANIC CHEMISTRY    Volume: 47    Issue: 22    Pages: 10288-10298   

Abstract: This work investigates the photoluminescence (PL) properties of the Cr³⁺-doped and Cr³⁺-pure fluoroelpasolites along the A(2)BMF(6) series and as a function of pressure. In particular, we focus on the variation of the crystal-field spectrum and the associated PL. The results are explained on the basis of the octahedral (CrF₆)(3-) complex subjected either to external pressure or the internal pressure exerted by different crystal hosts. We have established structural correlations between the crystal-field parameter 10Dq and the Cr-F distance, RCr-F, from which we have determined the local structure around the Cr3+ impurity, allowing the host material effect on the Cr-F bonds to be studied. As salient features, we show, first, a weak dependence of the first excitation energy, El, usually identified as 10Dq, with RCr-F as E-1 = KRCr-F-3.3 and, second, an increase of the Stokes shift upon RCr-F reduction or with increasing pressure. We associate this unusual behavior with the existence of state mixing among T-4(2g)(F), E-2(g)(G), and T-2(1g)(G) states in the first excitation band of Cr^3+. Finally, high-pressure experiments performed on Rb₂KCrF₆ indicate that the excited-state spin crossover, E-2(g)(G) <-> T-4(2g)(F), takes place around 7 GPa. The results indicate the suitability of the selected A(2)BMF(6):Cr3+ elpasolites to establish structural correlations between PL and RCr-F.

KeyWords Plus: NEAR-INFRARED LUMINESCENCE; D-D SPECTRUM; CHROMIUM PHOTOLUMINESCENCE; TEMPERATURE-DEPENDENCE; TRANSITION-METAL; SINGLE-CRYSTALS; ELECTRONIC-STRUCTURE; ABSORPTION-SPECTRA; FLUORIDE LATTICES; Cr³⁺ IONS

DOI: 10.1021/ic800606h

Author(s):Millot, Marius ,Broto, Jean-Marc, Gonzalez, Jesus

Source: PHYSICAL REVIEW B    Volume: 78    Issue: 15  Article Number: 155125    

Abstract: High-field Zeeman and Paschen-Back effects have been observed in single crystals of ruby submitted to hydrostatic pressure up to 10 GPa. A specific setup with a miniature diamond-anvil cell has been developed to combine high pressure and pulsed magnetic fields and to perform magnetophotoluminescence measurements. Careful analysis of low-temperature (4.2 and 77 K) photoluminescence spectra with a 56 T magnetic field applied along the c axis allows for the rectification of the assignment of observed emission lines to corresponding Zeeman-split levels. Besides, the intrinsic Zeeman-splitting factors of excited states reveal a linear pressure-induced increase. This enhancement is a signature of an increase in trigonal distortion induced by hydrostatic pressure. Moreover, spectra with magnetic field perpendicular to crystallographic c axis exhibit a Paschen-Back effect reflecting the progressive alignment of Cr³⁺ ions spin along the applied field. However, no pressure modification is observed in this compound, contrarily to the Heisenberg-to-Ising spin character pressure-induced transition observed in alexandrite.

KeyWords Plus: DIAMOND-ANVIL CELL; HIGH MAGNETIC-FIELDS; ABSORPTION SPECTRA; LINE SHIFTS; R-LINES; AL2O₃; Cr^-3; KBAR

DOI: 10.1103/PhysRevB.78.155125

Author(s): Millot, Marius, George, Sylvie, Broto, Jean-Marc, Couzinet, Bernard, Chervin, Jean-Claude, Polian, Alain, Power, Chrystian, Gonzalez, Jesus

Source: HIGH PRESSURE RESEARCH    Volume: 28    Issue: 4    Pages: 627-631

Abstract: We present here a new diamond anvil cell designed to combine high hydrostatic pressure up to 10GPa, non-destructive, pulsed magnetic fields up to 60T and low temperature as low as 2K. Both optical and transport measurements are now achievable. Magneto-photoluminescence set-up under pressure is described in great detail. We report photoluminescence experiments on oriented ruby single crystals exhibiting direct observation of the intrinsic Zeeman splitting of excited states under pressure.

Author Keywords: high pressure; high magnetic field; luminescence; diamond anvil cell; ruby

KeyWords Plus: SUPERCONDUCTIVITY

DOI: 10.1080/08957950802510554

Author(s): Nataf, Lucie, Rodriguez, Fernando, Valiente, Rafael

Source: HIGH PRESSURE RESEARCH    Volume: 28    Issue: 4    Pages: 553-558    

Abstract: This work investigates the Co2⁺⁽Td) electronic structure and photoluminescence (PL) in MgAl2O4:Co2+ and their variation with pressure by time-resolved spectroscopy. The absorption spectra and the pressure and temperature dependences of the emission band at 663nm (=120ns, at ambient conditions) can be explained on the basis of a configurational energy model, which provides an interpretation of the electronic structure and the excited-state phenomena. We show that there is an excited-state crossover [4T 1(P)2E (G)] at ambient pressure, which is responsible for the evolution of the emission spectrum from a broadband emission between 300 and 100K to a narrow-peak emission at 6K. The pressure, instead of enhancing excited-state crossover phenomena, reduces PL and suppresses it (PL quenching) above 6GPa. We explain such variations in terms of pressure-induced non-radiative crossover relaxation to lower excited states: 2E (G)4T 1(F). The variation of PL intensity and its associated lifetime with pressure supports the proposed interpretation.

Author Keywords: Co²⁺Td; MgAl₂O₄; excited-state crossover; time-resolved spectroscopy

KeyWords Plus: SATURABLE ABSORBER; TETRAHEDRAL CO²⁺; MGAL₂O₄; ABSORPTION; CRYSTAL; SPINEL; GLASS; IONS

DOI: 10.1080/08957950802510562

Author(s): Sanz-Ortiz, Marta N., Rodriguez, Fernando, Demazeau, Gerard

Source: HIGH PRESSURE RESEARCH    Volume: 28    Issue: 4    Pages: 571-576    

Abstract: This work investigates the electronic structure of CoF3 by optical absorption and its relationship with the ground-state spin and the Jahn-Teller (JT) effect exhibited by Co3+(d6) in octahedral coordination (CoF₆)^3-. The results are compared with other Co³⁺ fluorides and oxides, where Co3+ is high spin (HS) and low spin (LS), respectively. In CoF3 we detect an absorption band with a doublet structure at 1.89eV (657nm) and 1.45eV (855nm) at 2.5GPa, which is associated with the T e JT effect on the 5T2 HS ground state. Absorption measurements under pressure have been carried out around the HS-LS (5T2 1A1) spin crossover transition in the 0-20 GPa range. We show that the JT effect is stable in a wide pressure range. No sign of intermediate spin (3T1, 2 states) is observed in the explored pressure range.

Author Keywords: Co³⁺; CoF₃; spin crossover; intermediate spin; Jahn-Teller

DOI: 10.1080/08957950802552119

Author(s): Sanz-Ortiz, M. N., Rodriguez, F., Hernandez, I., Valiente, R., Kueck, S.

Source: JOURNAL OF LUMINESCENCE    Volume: 128    Issue: 5-6    Pages: 721-724    

Abstract: This work investigates phase transition (PT) and excited-state-crossover (ESCO) effects on the photoluminescence (PL) properties of LiCaAlF6:Cr³⁺. The structural requirements for changing the Cr³⁺ PL behavior from a broad-band emission at 1.59eV (781 nm) at ambient conditions, to ruby-like narrow-line emission at 1.87eV (663 nm) are analyzed in the 0-35 GPa range. We report a PL study on LiCaAlF₆:Cr³⁺ by means of time-resolved emission as a function of pressure and temperature. In particular we focus on the PL variations occurring around the pressure-induced trigonal-to-monoclinic first-order PT in LiCaAlF₆ at 7 GPa. (C) 2007 Elsevier B.V. All rights reserved.

Author Keywords: LiCaAlF6; Cr3+; high-pressure; time-resolved spectroscopy; excited state crossover

DOI: 10.1016/j.jlumin.2007.10.027

Author(s): Hernandez, I., Rodriguez, F, Hochheimer, H. D.

Source: JOURNAL OF LUMINESCENCE    Volume: 128    Issue: 5-6    Pages: 1007-1009   

Abstract: A novel two-color photoluminescence (PL) is found in MnF₂ at room temperature (RT) under high pressure. In contrast to the low-temperature PL, the observation of room-temperature PL is unusual in transition-metal concentrated materials like MnF₂ since the de-excitation process at RT is fully governed by energy transfer to non-radiative centers. We show that room-temperature MnF₂ emissions originate in the pressure-induced cotunnite phase. Both the nine-fold Mn²⁺ coordination and the Mn-F-Mn exchange pathway inhibit exciton migration among Mn²⁺ favoring two-band PL at RT. The electronic structure and the excited-state dynamics are investigated by time-resolved emission and excitation spectroscopies under pressure. The two PL bands at 2.34 and 1.87eV above 15 GPa are assigned to Mn2+ emissions arising from two distinct Mn²⁺ centers formed in the MnF2 high-pressure phase. The microscopic origin of the two-color PL is analyzed in terms of exciton dynamics in the MnF₂ cotunnite structure. (C) 2007 Elsevier B.V. All rights reserved.

Author Keywords: MnF2; high-pressure; time-resolved spectroscopy; energy transfer

DOI: 10.1016/j.jlumin.2007.10.030

Author(s): del Corro, Elena, González, Jesús, Taravillo, Mercedes, Flahaut, Emmanuel, Baonza, Valentín G.

Source: NANO LETTERS    Volume: 8    Issue: 8    Pages: 2215-2218    

Abstract: We investigated the pressure dependence of the Raman frequencies and intensities of the D and G bands of double-wall carbon nanotubes under strong uniaxial conditions. Using moissanite anvils, we observed for the first time the evolution of the D band under extreme stress/pressure conditions. We find that the difference between D and G frequencies remains constant over the whole stress range. In addition, we observe that double-wall carbon nanotubes behave elastically up to the maximum uniaxial stress reached in our experiments, which is estimated to be about 12 GPa.

KeyWords Plus: HIGH-PRESSURE; SPECTROSCOPY; SCATTERING; BUNDLE

DOI: 10.1021/nl080760o

Author(s): del Corro, Elena, Gonzalez, Jesus, Taravillo, Mercedes, Escoffier, Walter, Baonza, Valentin G.

Source: HIGH PRESSURE RESEARCH    Volume: 28    Issue: 4    Pages: 583-586    

Abstract: The combination of Raman spectroscopy and high-pressure techniques provides a unique method for studying the mechanical and structural response of carbon-based materials. Existing Raman studies on graphite under pressure were restricted to the analysis of the so-called G band, because the Raman signal from the diamond anvils overlaps other characteristic Raman features, like the D band. Here, we present a Raman spectroscopy study of highly oriented pyrolytic graphite (HOPG) under uniaxial stress using moissanite anvils. The use of moissanite has allowed us to observe, for the first time, the evolution of the D band under extreme compression. We have employed several excitation wavelengths: 632.8, 532.0 and 488.0nm, to further study dispersion effects. Our results have important implications on the interpretation of high-pressure Raman results on several families of carbon-based compounds.

Author Keywords: highly oriented pyrolytic graphite; Raman spectroscopy; uniaxial stress; moissanite anvil cell

KeyWords Plus: RAMAN; 1ST-ORDER; PRESSURE

DOI: 10.1080/08957950802444796

Author(s): Aguado, Fernando, Rodriguez, Fernando, Nunez, Pedro

Source: PHYSICAL REVIEW B    Volume: 76    Issue: 9  Article Number: 094417    

Abstract: The interplay between the orbital ordering and the spin state in Jahn-Teller Mn³⁺ governing the optical, magnetic, and transport properties in the layered CsMnF₄ perovskite is investigated. Such electronic effects are strongly coupled to the lattice and thus can be modified by external pressure. However, there is very little understanding of the structural conditions which are required to attain spin crossover in connection with the electronic structure of Mn^3+. The distortion, spin state, and tilting of (MnF6)(3-) octahedra in the insulating ferromagnet CsMnF₄ are jointly studied by high-pressure optical spectroscopy. The insulating character of CsMnF₄ allowed us to explore the electronic structure associated with the 3d levels of Mn3+ in the 0-46 GPa pressure range, an information which is obscured in most oxides due to metallization at high pressure. We show that the spin-crossover transition, related to the spin change, S = 2 -> 1, in Mn3+, takes place at 37 GPa with the simultaneous suppression of the axially elongated distortion associated with the Jahn-Teller effect. The wide stability pressure range of the Jahn-Teller distortion and high-spin state is explained in terms of crystal-field models including the Jahn-Teller stabilization energy. On this basis, we discuss the interplay between spin transition and Jahn-Teller effect comparing present findings with other results attained in Mn3+, Ni3+, and Co3+ systems.

KeyWords Plus: OPTICAL-PROPERTIES; METAL-INSULATOR; LOWER MANTLE; STATE; NI3+

DOI: 10.1103/PhysRevB.76.094417

Author(s): Aguado, F., Rodriguez, F., Valiente, R., Hanfland, M., Itie, J. P.

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 19    Issue: 34  Article Number: 346229    

Abstract: This work investigates the effect of pressure on the Jahn-Teller distortion (JTD) associated with the axially elongated CuCl6 octahedra in the A(2)CuCl(4) perovskite layer (A: Rb, CH3NH3, C2H5NH3, C3H7NH3). The aim is to elucidate whether pressure favours disappearance of the JTD in the antiferrodistortive (AFD) structure exhibited by Cu2+ within the layers or whether it induces tilts of the CuCl6 octahedra preserving the molecular distortion associated with the JT effect. We have carried out x-ray absorption (XAS) and x-ray diffraction (XRD) experiments under pressure along the compound series, whose interlayer distances at ambient pressure vary from 7.77 to 12.33 angstrom. The use of both XAS and XRD techniques allows us a complete local- and crystal-structure characterization in Rb2CuCl4 as a function of pressure in the 0-16 GPa range. We show that pressure reduces the axial (long) and equatorial (short) Cu-Cl distances, R-ax and R-eq, as well as the intralayer and interlayer Cu-Cu distances, d(Cu-Cu) and d(inter). Interestingly, the variation of R-ax is an order of magnitude bigger than that of the corresponding R-eq, yielding a reduction of the JTD. However, no evidence of JTD suppression has been observed below 16 GPa. Pressure-induced CuCl6 tilting preserves the JTD in a wide pressure range. Estimates based on structural data suggest that JT suppression would occur at about 40 GPa.

KeyWords Plus: NEUTRON-SCATTERING MEASUREMENTS; HEISENBERG-FERROMAGNET K2CUF4; MAGNETIC-SUSCEPTIBILITY; POWDER DIFFRACTION; CHARGE-TRANSFER; TRANSITION; (C2H5NH3)(2)CUCL4; DISAPPEARANCE; MOMENT; SERIES

DOI: 10.1088/0953-8984/19/34/346229

Author(s): Hernandez, Ignacio; Rodriguez, Fernando

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 19    Issue: 35  Article Number: 356220    

Abstract: This work presents a correlated structural and spectroscopic study on ball-milled MnF₂. The aims are to produce impurity-lean particles through particle-size reduction leading to room-temperature photoluminescence (PL) and to modify the electronic states of the emitting centres. Despite non-radiative centres being still present, the PL quenching temperature was increased nearly 80 K, from 120 to 200 K, following this method. Milled MnF₂ has particle sizes down to several nanometres, and structural changes from the initial alpha-TiO₂ structure to the alpha-PbO₂ phase. Milling favours the presence of adsorbed water on the nanoparticle surface. Time-resolved spectroscopy indicates that the nanoparticle PL consists of a significantly inhomogeneous broadened band with respect to the initial MnF₂ PL. The temperature dependence of the lifetime measured at different wavelengths of the emission spectrum indicates the presence of several PL centres, the population of which is controlled by exciton migration and trapping. The widespread occurrence of emitting centres is explained in terms of milling-induced strains, the coexistence of two different structural phases, and the presence of adsorbed water molecules.

KeyWords Plus: ENERGY-TRANSFER; MAGNETIC-STRUCTURE; CRYSTALS; FLUORESCENCE; LUMINESCENCE; IONS; TEMPERATURE; TRANSITIONS; PRESSURE

DOI: 10.1088/0953-8984/19/35/356220

Author(s): Hernandez, I., Rodriguez, Fernando, Hochheimer, H. D.

Source: PHYSICAL REVIEW LETTERS    Volume: 99    Issue: 2  Article Number: 027403    

Abstract: A novel two-color photoluminescence (PL) is found in MnF2 at room temperature under high pressure. Contrary to low-temperature PL, PL at room temperature is unusual in transition-metal concentrated materials like MnF2, since the deexcitation process at room temperature is fully governed by energy transfer to nonradiative centers. We show that room-temperature PL in MnF2 originates from two distinct Mn2+ emissions in the high-pressure cotunnite phase. The electronic structure and the excited-state dynamics are investigated by time-resolved emission and excitation spectroscopy at high pressure.

KeyWords Plus: ENERGY-TRANSFER; OPTICAL SPECTROSCOPY; CRYSTALS; FLUORESCENCE; LUMINESCENCE; IONS; SYSTEMS; MN-2+; KMGF3; LASER

DOI: 10.1103/PhysRevLett.99.027403

Author(s): Rodriguez, F., Aguado, F, Valiente, R., Hanfland, M.,Itie, J. P.

Source: PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS    Volume: 244    Issue: 1    Pages: 156-161    

Abstract: This work investigates the variation of the first charge-transfer (CT) transition with pressure in the A(2)CuCl(4) layered perovskites, in terms of structural changes, in particular, the local structure of the (CuCl6)(4-) octahedra. We show that pressure partially reduces the Jahn-Teller distortion. The decrease of the axial Cu-Cl distance, R-ax, is an order of magnitude bigger than the decrease of the equatorial distance, R-eq, but the in-plane Cu-Cu distance reduction is almost twice that the sum of R-ax and R-eq, thus indicating pressure-induce tilts. This phenomenon avoids suppression of the JT effect, keeping the (CuCl6)(4-) axial distortion above 10 GPa. Both the partial reduction of the JT distortion and tiliting can account for the CT redshift induced by pressure. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

KeyWords Plus: STRUCTURAL CORRELATIONS; HYDROSTATIC-PRESSURE; CU2+; (C2H5NH3)(2)CDCL4CU2+; SPECTROSCOPY; SPECTRA; SYSTEMS

DOI: 10.1002/pssb.200672506

Author(s): Valiente, Rafael, Garcia-Lastra, Juan M. , Garcia-Fernandez, Pablo, Garcia-Revilla, Sara, Wenger, Oliver S.

Source: EUROPEAN JOURNAL OF INORGANIC CHEMISTRY    Issue: 36    Pages: 5735-5742    

Abstract: The combination of spectroscopic and computational methods has been employed to explore the origin of the luminescence in the red and yellow forms of Pt(bpy)Cl-2. The luminescence of the red (linear-chain) form of Pt(bpy)Cl-2 (bpy 2,2′-bipyridine) has been measured as a function of hydrostatic pressure up to 65 kbar. The luminescence band maximum is redshifted (-147 cm(-1) kbar(-1)), and the intrachain Pt-Pt distance decreases from 3.46 to 3.25 angstrom between ambient pressure and 17.7 kbar. Strong discontinuities in the optical properties at 17.7 kbar were interpreted in terms of a crystallographic phase transition from the red to a denser yellow form of Pt(bpy)Cl-2. First-principles calculations based on density functional theory were used to study the red to yellow phase transformation. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).

Author Keywords: phase transition; high-pressure chemistry; DFT calculations; platinum(II); metal-metal interactions

KeyWords Plus: ALPHA-DIIMINE LIGANDS; PLATINUM(II) COMPLEXES; ELECTRONIC-SPECTRA; EXCITED-STATES; SQUARE-PLANAR; LUMINESCENCE; DENSITY; PHOTOPHYSICS; DEPENDENCE; ENERGIES

DOI: 10.1002/ejic.200700060

Author(s): Aguado, F., Rodriguez, F.

Source: HIGH PRESSURE RESEARCH    Volume: 26    Issue: 4    Pages: 319-323 

Abstract: This work deals with structural correlation studies in Jahn-Teller (JT) systems of Cu2+ and Mn3+ in the layer perovskites A(2)CuCl(4) and AMnF(4). The aim is to explore the influence of the JT effect on the materials’ properties as well as its variation with pressure. We present structural studies carried out in the layer perovskites AMnF(4) by optical spectroscopy under pressure in order to explore whether pressure reduces the JT distortion or induces tilting phenomena of the JT octahedra (MnF6)(3-). The influence of the JT effect on the stabilization of the ground-state spin, either high-spin (HS) or low-spin (LS), in Mn3+ is analyzed. This knowledge is important to understand and, eventually predict, magnetic, electrical and optical properties in connection with HS-LS phenomena. The results are compared with previous structural studies performed in the (CuCl6)(4-) JT octahedra formed in A(2)CuCl(4).

Author Keywords: Mn3+; CU2+; Jahn-Teller effect; spectroscopy under pressure; layer perovskite

KeyWords Plus: TRANSITION

DOI: 10.1080/08957950601104427

Author(s): Garcia-Revilla, S., Valiente, R.

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 18    Issue: 49    Pages: 11139-11148    

Abstract: Single crystals of CsCdBr3 doped with Eu2+ at different concentrations were grown by the Bridgman technique and studied by polarized variable-temperature optical absorption and luminescence spectroscopy for different excitation wavelengths. Absorption bands of different natures were assigned in terms of concentration behaviour and temperature dependent photoluminescence. The low-energy structured band was due to f -> d transitions within the Eu2+ electronic configuration. The proximity of the high-energy d levels to the conduction band induces self-ionization of Eu2+. The high-energy absorption bands were assigned to self-trapped excitons.

KeyWords Plus: RARE-EARTH IONS; INORGANIC-COMPOUNDS; CRYSTALS; SPECTROSCOPY; EU2+; SPECTRA; LUMINESCENCE; EXCITATION; CENTERS

DOI: 10.1088/0953-8984/18/46/008

Author(s):  Hernandez, I., Rodriguez, F.,  Hochheimer, H. D.

Source: HIGH PRESSURE RESEARCH    Volume: 26    Issue: 4    Pages: 331-334    

Abstract: This work presents a time-resolved spectroscopic study of MnF₂ as grown and milled into nanoparticles. We find that high pressure (HP) induces photoluminescence (PL) at room temperature (RT) above 13 GPa in MnF₂ single crystals. RT-PL occurs as a consequence of the inhibition of excitation migration processes among Mn2+ ions resulting from the HP-induced structural phase transition (SPT) to the alpha-PbCl2-type phase. This SPT involves important changes in the Mn-F-Mn exchange paths yielding radiative de-excitation in Mn2+, and thus also PL. As a salient result, we have found an increase of the PL-quenching temperature in nanosized milled MnF₂.

Author Keywords: photoluminescence; high pressure; nanoparticles; MnF₂; time-resolved spectroscopy; phase transition

DOI: 10.1080/089579506001104096

Author(s): Sanz-Ortiz, M. N., Rodriguez, E., Hernandez, I., Valiente, R., Kuecks, S.

Source: HIGH PRESSURE RESEARCH    Volume: 26    Issue: 4    Pages: 345-348    

Abstract: This work investigates pressure-induced excited-state-crossover phenomena in Cr3+-doped LiCaAlF6 at room temperature. The aim is to find the structural requirements yielding the change in the Cr3+ photoluminescence (PL) behaviour from a single broadband emission, which is located at 1.62eV (765 nm) at ambient conditions, to a ruby-like narrow-line emission at 1.87 eV (662 nm). For this purpose, emission and excitation spectroscopy as well as lifetime measurements as a function of pressure were performed in the 0-35 GPa range. Interestingly, we have been able to transform the broadband Cr3+ PL at ambient pressure into a ruby-like emission at 28 GPa. This behaviour, together with the variations of the E-2 and T-4(2) excited-state energies, and the PL lifetime with pressure, can be explained on the basis of the electron-ion coupling associated with the T-4(2) and E-2 states.

Author Keywords: Spectroscopy; high pressure; photoluminescence; Cr3+; excited-state crossover

KeyWords Plus: SPECTROSCOPY

DOI: 10.1080/08957950601104237

Author(s): Garcia-Revilla S, Gerner P, Gudel HU, Valiente R

Source: PHYSICAL REVIEW B    Volume: 72    Issue: 12  Article Number: 125111    

Abstract: Near-infrared excitation of Yb3+ F-2(7/2)->F-2(5/2) at 10 600 cm(-1) in the linear chain CsMBr3:Ni2+,Yb3+ (M=Cd,Mg) structure leads to green luminescence from Yb3+ pairs and red Ni2+ upconversion luminescence at cryogenic temperatures. The broad red upconversion luminescence is assigned to the T-1(2g)->(3)A(2g) transition of Ni2+. The upconversion excitation spectrum indicates that the active mechanism in these systems involves both Yb3+ and Ni2+ ions. Lifetime and power dependence data allow one to identify a ground-state-absorption (GSA) energy-transfer upconversion (ETU) as the underlying upconversion mechanism for the Yb3+-Ni2+ system in doubly doped CsCdBr3 and CsMgBr3. A less efficient single-ion GSA excited-state-absorption (ESA) process within Ni2+ is also observed in the title compounds. However, whereas the GSA/ESA process requires an excitation energy above 11 200 cm(-1), it is possible to sensitize the Ni2+ upconversion via Yb3+ excitation at energies around 10 200 cm(-1) and around 10 600 cm(-1) in CsCdBr3 and CsMgBr3. It is shown that only those Ni2+ ions neighboring to Yb3+ ions are relevant for the upconversion induced by Yb3+ excitation.

KeyWords Plus: HIGHER-EXCITED-STATE; YB3+ DOPED CSMNCL3; CR3+ E-2 EMISSION; TRANSITION-METAL; LUMINESCENCE PROPERTIES; CHLORIDE LATTICES; NEAR-IR; NONRADIATIVE PROCESSES; EXCITATION AVALANCHE; COUPLED PAIRS Visible Ni²⁺ upconversion luminescence in Ni²⁺, Yb³⁺ co-doped CsCdBr₃KeyWords Plus: HIGHER-EXCITED-STATE; YB3+ DOPED CSMNCL3; CR3+ E-2 EMISSION; TRANSITION-METAL; LUMINESCENCE PROPERTIES; CHLORIDE LATTICES; NEAR-IR; NONRADIATIVE PROCESSES; EXCITATION AVALANCHE; COUPLED PAIRS

DOI: 10.1103/PhysRevB.72.125111

Author(s): Garcia-Revilla S, Gerner P, Wenger OS, Gudel HU, Valiente R

Source: CHEMICAL PHYSICS LETTERS    Volume: 401    Issue: 4-6    Pages: 492-496    

Abstract: Excitation around 10200 and 10600 cm(-1) into the absorption peaks of Yb (3+) leads to visible Ni2+ T-1(2g)–>(3)A(2g) upconversion luminescence in the doubly doped CSCdBr3: Ni2+, Yb (3+) at cryogenic temperatures. The excitation spectrum indicates that ail i2l 3Yb(3+) absorption step is involved in the upconversion process leading to visible Ni2+ luminescence. This process via Yb3+ excitation is more efficient than the upconversion mechanism involving only Ni2+, which is observed in the title compound for excitation energies above 11200 cm(-1). Upconversion lifetimes point towards an energy transfer upconversion mechanism for the Ni2+/Yb3+ system in CsCdBr3 (C) 2004 Elsevier B.V. All rights reserved.

KeyWords Plus: CR3+ E-2 EMISSION; TRANSITION-METAL; NEAR-IR; LATTICES; GREEN; MN2+; EXCITATION; HALIDES; CSMNCL3

DOI: 10.1016/j.cplett.2004.11.115

Author(s): Aguado F, Rodriguez F, Valiente R, Itie JP, Munsch P

Source: PHYSICAL REVIEW B    Volume: 70    Issue: 21  Article Number: 214104    

Abstract: In this work we investigate the variation of the local structure around Cu2+ as well as the crystal structure in Rb2CuCl4(H2O)(2) through x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD) as a function of pressure. We show that the application of pressure induces a local structural change in the Jahn-Teller (JT) CuCl4(H2O)(2)(2-) complex from an axially elongated complex to a compressed one, yielding disappearance of the JT distortion related to the four in-plane Cl- ligands, which are responsible for the antiferrodistortive structure displayed by the crystal at ambient pressure. According to the Pseudo-Jahn-Teller (PJT) theory (electron-phonon coupling Ecircle timese), the presence of water ligands enhances the JT release at pressures well below the metallization pressure. The results are compared with recent pressure experiments on A(2)CuCl(4) layered perovskites and heteronuclear CuCl4L2 complex series, L :Cl–>H2O–>NH3, and explained on the basis of the PJT model.

KeyWords Plus: PARAMAGNETIC-RES SPECTRA; OPTICAL-PROPERTIES; STRUCTURAL CORRELATIONS; TEMPERATURE-DEPENDENCE; COMPRESSED OCTAHEDRA; CHARGE-TRANSFER; EPR-SPECTRUM; CRYSTAL; LAYER; STATE

DOI: 10.1103/PhysRevB.70.214104

Author(s): Aguado F, Rodriguez F, Valiente R, Senas A, Goncharenko I

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 16    Issue: 12    Pages: 1927-1938    

Abstract: This work investigates the magnetic structure of Rb2CuCl4 as a function of pressure and temperature using neutron diffraction. As in most A(2)CuCl(4) layered perovskites, there is a 2D ferromagnetic order within the layers. This behaviour is due to the Jahn-Teller (JT) antiferrodistortive structure of the CuCl6 units. Rb2CuCl4 undergoes a 3D magnetic transition at T-N = 16 K, which mainly depends on the weak antiferromagnetic interlayer interaction. The pressure slightly increases T-N, as partial derivativeT(N)/partial derivativeP = 0.13 K kbar(-1). This behaviour is interpreted in terms of pressure-induced tilts and reduction of interlayer distance, both effects increasing the anti ferromagnetic exchange coupling between layers. The results are compared with previous magnetic studies under chemical and hydrostatic pressure along layered perovskites series of [CnH2n+1NH3](2)CuCl4 (n = 1-3) and BMnF4 (B = Li, Na, K, Rb, Tl, Cs and NH4) involving JT ions of Cu2+ and Mn3+, respectively. We show that the ratio of the interlayer to intralayer coupling, and thus the nature of the magnetic order, can be tuned by chemical or hydrostatic pressure along the A(2)CuCl(4) series. The present findings stress the relevance of octahedral tilts on the magnetic behaviour of layered perovskites.

KeyWords Plus: 2-DIMENSIONAL HEISENBERG-FERROMAGNET; NEUTRON-SCATTERING MEASUREMENTS; ELECTRON-PARAMAGNETIC-RES; CRYSTAL-STRUCTURE; SUPEREXCHANGE INTERACTION; HYDROSTATIC-PRESSURE; PLANAR FERROMAGNET; SUSCEPTIBILITY; K2CUF4; EXCHANGE

DOI: 10.1088/0953-8984/16/12/003

Author(s): Reinhard C, Gerner P, Rodriguez F, Garcia-Revilla S, Valiente R, Gudel HU

Source: CHEMICAL PHYSICS LETTERS    Volume: 386    Issue: 1-3    Pages: 132-136    

Abstract: Yb3+ excitation in the near-infrared around 1 mum leads to green Mn2+.4T1 –>(6)A(1) luminescence in CaZnF4:Mn2+; Yb3+, ZwSiO(4):Mn2+; Yb3+ and SrZnCl4:Mn2+; Yb3+ at 15 K. In the former two compounds the green upconversion (UC) luminescence is visible by eye up to room temperature. In Cs3MnBr5:Mn2+; Yb3+ no UC luminescence is observed. The UC mechanism consists of a sequence of ground-state absorption and excited-state absorption steps. The UC efficiency at 15 K is of the order of 1% for laser excitation with 130 mW (f = 53 mm focusing lens). (C) 2004 Elsevier B.V. All rights reserved.

KeyWords Plus: CRYSTAL-STRUCTURES; LUMINESCENCE; CSMNCL3

DOI: 10.1016/j.cplett.2003.12.127

Author(s): Rodriguez F

Source: HIGH-PRESSURE CRYSTALLOGRAPHY   Book Series: NATO SCIENCE SERIES, SERIES II: MATHEMATICS, PHYSICS AND CHEMISTRY    Volume: 140    Pages: 341-352    

KeyWords Plus: CHARGE-TRANSFER; HYDROSTATIC-PRESSURE; CRYSTAL; SPECTRUM; SPECTROSCOPY; IMPURITIES; DEPENDENCE; DISTORTION; COMPLEXES; LATTICE

Reprint Address: Rodriguez, F (reprint author), Univ Cantabria, Fac Ciencias, DCITIMAC, E-39005 Santander, Spain

uthor(s): Aguado F, Rodriguez F, Nunez P

Source: PHYSICAL REVIEW B    Volume: 67    Issue: 20  Article Number: 205101

Abstract: This work investigates the optical absorption spectrum of the NaMnF4-layered perovskite and its variation with pressure. The spectrum basically consists of three broadbands located at 1.916, 2.263, and 2.817 eV, which correspond to the crystal-field (CF) transitions B-5(1g)–>(5)Gamma(i) (Gamma(i)=A(1g), B-2g, and E-g) with the Jahn-Teller- (JT-) distorted MnF63- complex (Mn3+ d(4) configuration). In addition, there are two spin-flip B-5(1g)–>B-3(1g) peaks at 2.397 and 2.890 eV, which are activated by the exchange mechanism. Their variation with pressure reveals that the JT energy does not change significantly with pressure: partial derivativeE(JT)/partial derivativeP=0.8 meV/GPa. Furthermore, the variation of the JT tetragonal splitting of the parent octahedral e(g) and t(2g), termed Delta(e) and Delta(t), respectively, clearly indicate that partial derivativeDelta(e)/partial derivativeP<&PARTIAL;&UDelta;(t)/&PARTIAL;P, although &UDelta;(e)&AP;4&UDelta;(t). The CF energies and their pressure shift are explained in terms of local structural changes within the MnF63- complex induced by pressure. The structural correlation analysis reveals that the reduction of the MnF63- JT distortion is smaller than the expected one on the basis of the crystal volume reduction, thus indicating tilt phenomena. This interpretation is supported by the decrease of in-layer Mn-F-Mn superexchange, such as is derived from the optical spectra. We demonstrate that the equatorial and axial distances decrease from 1.839 to 1.808 Angstrom and from 2.167 to 2.107 Angstrom, respectively, in the 0-10 GPa range.

KeyWords Plus: METAL-LIGAND DISTANCE; MAGNETIC-STRUCTURES; FERROMAGNET CSMNF4; OPTICAL-SPECTRUM; SINGLE-CRYSTAL; DEPENDENCE; PEROVSKITES; TRANSITION; FLUORIDES; 10DQ

DOI: 10.1103/PhysRevB.67.205101

Author(s): Aguado F, Rodriguez F, Valiente R, Itite JP, Munsch P

Source: HIGH PRESSURE RESEARCH    Volume: 23    Issue: 1-2    Pages: 181-186    

Abstract: This work investigates the pressure-induced variation of the local structure around Cu2+ as well as the crystal structure in Rb₂CuCl₄(H₂O)(2) through XAS and XRD techniques. The application of pressure induces a structural change in the Jahn-Teller (JT) CuCl4(H2O)(2)(2-) mplex from axially elongated to compressed. This change leads to the closing of the 2D JT distortion related to the four in-plane Cl- ligands, which are responsible for the antiferrodistortive structure displayed by the crystal. It is shown that the presence of water ligands enhances a JT release. Their associated axial ligand-field favours the occurrence of such a local structural transition below the metallization pressure. The results are compared with recent pressure experiments on A(2)CuCl(4) systems.

Author Keywords: Cu²⁺; CuCl₄(H₂O)(2); Jahn-Teller effect; XRD and XAS under pressure; layer perovskite

KeyWords Plus: CHARGE-TRANSFER; CRYSTAL; NH4CL; XANES; EDGE; CU

DOI: 10.1080/0895795031000147731

uthor(s): Guemez J, Valiente R, Fiolhais C, Fiolhais M

Source: AMERICAN JOURNAL OF PHYSICS    Volume: 71    Issue: 12    Pages: 1264-1267    

Abstract: A theoretical description of the sunbird, a drinking bird without any external liquid, is compared with experiment. The transient times and the periods of oscillation given by a simulation of the dynamics agree with the measured values. (C) 2003 American Association of Physics Teachers.

Reprint Address: Guemez, J (reprint author), Univ Cantabria, Dept Fis Aplicada, E-39005 Santander, Spain

DOI: 10.1119/1.1603273

Author(s): Hernandez I, Rodriguez F

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 15    Issue: 13    Pages: 2183-2195    

Abstract: This work investigates the photoluminescence (PL) properties of the cubic chloroperovskite NH4MnCl3. Like in most concentrated materials, the Mn2+ PL which is located at 2.10 eV at T = 10 K strongly depends on the temperature. Optical absorption (OA), emission, and excitation spectroscopy, as well as lifetime measurements, performed on NH4MnCl3 indicate that the PL is mainly intrinsic at T = 10 K and consists of a broad band located at 2.10 eV. Above this temperature, the PL gradually transforms to extrinsic PL due to exciton migration and subsequent trapping. Further temperature increase above 100 K yields transfer to killers of excitation which are responsible for the PL quenching, and hence the absence of PL at ambient conditions. The exciton traps are identified with perturbed Mn2+ sites with the effective activation energy of 52 meV, whilst the activation energy for energy transfer is 47 meV. The existence of these traps has been directly revealed by time-resolved spectroscopy. The detected intrinsic and extrinsic PL bands are displaced by 6 meV, which coincides with the activation energy difference between pure Mn2+ and trap Mn2+, as derived from temperature dependence studies of the lifetime T(T). Interestingly, a PL band at 1.82 eV is observed above 60 K. This band, which was initially associated with deeper excitation traps, actually corresponds to precipitates of MnCl2 inside NH4MnCl3. The correlation analysis performed on NH4MnCl3 using OA, PL, and lifetime data provides an estimate of the precipitate concentration of 0.3 mol %. The presence of two separated Mn2+ PL bands at different temperatures is a rather common phenomenon in concentrated materials such as AMnX(3) (A = NH4, Rb; X = Cl, F), and has been interpreted in terms of exciton transfer to deeper traps. The present finding stresses the relevance of an adequate structural characterization in dealing with PL in concentrated materials.

KeyWords Plus: OPTICAL-SPECTRUM; ENERGY-TRANSFER; CRYSTALS; TRANSITIONS; DEPENDENCE; DISTANCE; LUMINESCENCE; ABSORPTION; EXCITATION; DYNAMICS

Author(s): Hernandez I, Rodriguez F

Source: HIGH PRESSURE RESEARCH    Volume: 23    Issue: 1-2    Pages: 155-160

Abstract: This work investigates the surprising disappearance of the Mn2+ photoluminescence on passing from CaF2:Mn2+ to SrF2:Mn2+ or BaF2:Mn2+ using pressure spectroscopy. We show that the loss of luminescence in these systems is associated with non-radiative thermally activated processes whose activation energy and pre-exponential rates strongly depend on the crystal volume irrespective of the chemical composition. A salient feature is the PL enhancement observed in the cotunnite high-pressure phase along the series. This enhancement is explained in terms of the large volume reduction at the phase transition, as well as by the presence of low-symmetry crystal fields attained at the cation sites leading to an increase of the radiative transition rate by the electric-dipole mechanism.

Author Keywords: high-pressure spectroscopy; photoluminescence; radiationless process; Mn2+-doped fluorites; fluorite-cotunnite phase transition

KeyWords Plus: MN2+ IONS; PRESSURE; PHOTOLUMINESCENCE; ENHANCEMENT

DOI: 10.1080/0895795031000147777

Author(s): Rodriguez F, Aguado F

Source: JOURNAL OF CHEMICAL PHYSICS    Volume: 118    Issue: 24    Pages: 10867-10875  

Abstract: This work investigates the Jahn-Teller (JT) distortion in different Mn3+ fluoride series by optical absorption (OA) spectroscopy. The aim is to establish correlations between the local structure of the formed MnF63- derived from x-ray diffraction and the JT splitting associated with the parent octahedral E-5(g)(3z(2)-r(2),x(2)-y(2)) and T-5(2g)(xy,xz,yz) states, Delta(e) and Delta(t), obtained from the OA spectrum. A salient feature is the linear relation exhibited by both Delta(e) and Delta(t) with the tetragonal coordinate Q(theta) along the whole series. From these relations we derive suitable electron-ion coupling coefficients related to the E-5(g) and T-5(2g) states whose values play a key role in the exE and exT JT theory, respectively. The results of these correlations are applied to investigate the structural variations undergone by the two-dimensional compounds NaMnF4 and TlMnF4 under pressure using OA spectroscopy. Interestingly, the analysis carried out is relevant since it provides useful information on the Mn3+ local structure, a task that is difficult to achieve using extended x-ray-absorption fine structure under pressure due to the high absorption of the diamond anvils. We conclude that the effect of pressure in NaMnF4 is to reduce progressively the JT distortion of the complex, keeping its tetragonal symmetry. The pressure effects in TlMnF4 are more drastic, leading to pressure-induced structural phase transitions of low symmetry. At variance with NaMnF4, the high-pressure Mn3+ local structure seems to have significant rhombic distortions. (C) 2003 American Institute of Physics.

KeyWords Plus: MAGNETIC-STRUCTURES; CRYSTAL-STRUCTURE; FERROMAGNET CSMNF4; TRANSITION; SPECTRUM; FIELD; FLUOROMANGANATES(III); PEROVSKITES; DISTORTION; ABSORPTION

DOI: 10.1063/1.1569847

Author(s): Rodriguez F, Hernandez I, Moreno M, Alcala R

Source: JOURNAL OF CHEMICAL PHYSICS    Volume: 119    Issue: 16    Pages: 8686-8694   

Abstract: This work investigates the photoluminescence (PL) properties of Mn2+-doped fluorites in CaF2, BaF2, SrF2, and in solid solutions Ca1-xSrxF2. In particular, we focus on the radiationless processes leading to the surprising disappearance of the Mn2+ PL on passing from CaF2: Mn2+ to SrF2: Mn2+ or BaF2: Mn2+. For this purpose emission, excitation, lifetime, and time-resolved spectroscopy as a function of pressure and temperature are carried out in these compounds as well as in the Ca1-xSrxF2:Mn2+ (x=0-1) series using pressure spectroscopy. We show that the quenching of PL in these systems is associated with nonradiative thermal activated processes whose activation energy and pre-exponential rates strongly depend on the crystal volume irrespective of the chemical composition of the host crystal. A salient feature of this work deals with the increase of activation energy induced by pressure, whose variation with the lattice parameter is given by E-a(eV)=1.02-2.64 (a-5.46), with a(CaF2)=5.46 Angstrom. It leads to a PL quantum yield enhancement, which favors appearance of Mn2+ PL even in the non-PL systems SrF2:Mn2+ and BaF2:Mn2+. Furthermore, the activation energy mainly depends on the crystal volume per molecule irrespective of the crystal structure or the local symmetry around the impurity. In this way, the relevance of the fluorite-to-cotunnite phase transition to enhance PL is analyzed. This enhancement is explained in terms of the large volume reduction at the phase transition, as well as by the presence of low-symmetry crystal fields attained at the cation site yielding an increase of the radiative transition rate by the electric-dipole mechanism.(C) 2003 American Institute of Physics.

KeyWords Plus: MN²⁺ IONS; CRYSTALS; BAF₂; LUMINESCENCE; INSTABILITY; ENHANCEMENT; TRANSITIONS; SUPPRESSION; DEPENDENCE; DISTANCE

DOI: 10.1063/1.1611874

Author(s): Salley GM, Valiente R, Gudel HU

Source: PHYSICAL REVIEW B    Volume: 67    Issue: 13  Article Number: 134111    

Abstract: Green Tb3+ D-5(4)–>F-7(J) luminescence visible by eye is observed under near-infrared laser excitation. Optical spectroscopic techniques including absorption, luminescence, and excitation spectroscopy are used to characterize this upconversion (UC) luminescence. The Tb3+ UC luminescence is present for all temperatures within a range from 10 to 300 K, and gains intensity by three orders of magnitude between 10 and 300 K. For Tgreater than or equal to100 K the dominant upconversion mechanism is the cooperative sensitization of Tb3+ by two Yb3+ ions. In this temperature regime the Tb3+ UC luminescence dominates the visible (VIS) spectrum for all near-infrared (NIR) excitations, resulting in the characteristic green luminescence. At 10 K, the color of the luminescence changes from green to blue, depending on the excitation wavelength corresponding to the dominance of Tb3+ UC luminescence or the Yb3+-Yb3+ cooperative pair luminescence. Two color excitation spectroscopy is performed to directly observe an excited state absorption (ESA) step in the Tb3+ UC luminescence excitation spectrum at 10 K. This allows the unambiguous assignment of a type of ground state absorption/excited state absorption (GSA/ESA) mechanism responsible for the upconversion in this system at 10 K. We explain this cooperative interaction in the framework of an Yb3+-Tb3+ exchange-coupled dimer. An energy level diagram for this dimer is presented. Excitation into dimer levels around 12000-14500 cm(-1), where neither Yb3+ nor Tb3+ single ions have levels, leads to Yb3+ luminescence at 10 K. For laser excitation, 53 W/mm(2), resonant with an ESA transition a VIS/NIR photon ratio of 2.7(10)(-5) is found at 10 K.

KeyWords Plus: LUMINESCENCE UP-CONVERSION; YB3+ DOPED CSMNCL3; EXCHANGE INTERACTIONS; NEUTRON SPECTROSCOPY; TRANSITION-METAL; ENERGY-TRANSFER; NEAR-IR; CS3YB2BR9; SYSTEMS; MECHANISMS

DOI: 10.1103/PhysRevB.67.134111

Author(s): Hernandez, I., Rodriguez, F

Source: PHYSICAL REVIEW B    Volume: 67    Issue: 1  Article Number: 012101    

Abstract: This work reports an effective way for inducing room temperature photoluminescence (PL) in Mn2+-doped BaF2 and SrF2 using high-pressure techniques. The aim is to understand the surprising PL behavior exhibited by Mn2+ at the cubal site of the fluorite structure. While Mn2+-doped CaF2 shows a green PL with quantum yield close to 1 at room temperature, Mn2+-doped MF2 (M=Ba,Sr) is not PL either at room temperature (SrF2) or at any temperature (BaF2) at ambient pressure. We associate the loss of Mn2+ PL on passing from CaF2 to SrF2 or BaF2 with nonradiative multiphonon relaxation whose thermal activation energy decreases along the series CaF2–>SrF2–>BaF2. A salient feature of this work deals with the increase of activation energy induced by pressure. It leads to a quantum yield enhancement, which favors PL recovery. Furthermore, the activation energy mainly depends on the crystal volume per molecule irrespective of the crystal structure or the local symmetry around the impurity. In this way, the relevance of the fluorite-to-cotunnite phase transition is analyzed in connection with the PL properties of the investigated compounds. The PL spectrum and the corresponding lifetime are reported for both structural phases as a function of pressure.

KeyWords Plus: MN2+ IONS; CRYSTALS; LUMINESCENCE; INSTABILITY; TRANSITIONS; DEPENDENCE; DISTANCE

Reprint Address: Rodriguez, F (reprint author), Univ Cantabria, Fac Ciencias, Dept Ciencias Tierra & Fis Mat Condensada, E-39005 Santander, Spain

DOI: 10.1103/PhysRevB.67.012101

Author(s): Garcia-Revilla S, Rodriguez F, Valiente R, Pollnau M

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 14    Issue: 3    Pages: 447-460    

Abstract: This work investigates the effect of hydrostatic pressure on the excitation and emission spectra, as well as on the lifetime, of Al2O3:Ti3+ at room temperature. The aim is to establish correlations between the pressure-induced band shifts and the corresponding local structural changes undergone by the TiO6 complex. A blue-shift of 8.52 and 6.86 cm(-1) kbar(-1) was found for the lower (E-1) and upper (E-2) energy components of the excitation band at 17 760 and 20500 cm(-1), respectively, and blue-shifts of 5.93 and 5.40 cm(-1) kbar(-1) for the two overlapping bands of the emission spectrum located at 12 680 and 14210 cm(-1). We explain these results on the basis of a reduction of the TiO6 Jahn-Teller distortion upon increasing the pressure, In contrast, the increase of the overall Stokes shift, which is mainly associated with electron-vibration coupling to the totally symmetric a(1g) vibration, is explained by the increase of the excited-state stabilization energy, S(a1g)homega(a1g), with increasing pressure.

The luminescence lifetime is also found to be pressure dependent, varying from 2.6 mus at ambient conditions to 3.2 mus at 80 kbar. This increase is caused by a diminution of the transition oscillator strength that is related to the odd-vibration assistance mechanism. The softening of the transition mechanism is interpreted in terms of the blue-shift experienced by the O2- –> Ti3+ charge-transfer transition energy upon increasing the pressure.

KeyWords Plus: CHROMIUM PHOTOLUMINESCENCE; LOCAL COMPRESSIBILITIES; STOKES SHIFT; DEPENDENCE; TRANSITIONS; ABSORPTION; ELPASOLITES; SAPPHIRE; DISTANCE; SPECTRA

Author(s): Aguado F, Rodriguez F, Nunez P

Source: HIGH PRESSURE RESEARCH    Volume: 22    Issue: 1    Special Issue: Sp. Iss. SI    Pages: 121-126    

Abstract: This work investigates the Optical Absorption spectrum of the AMnF(4) layer perovskites of Mn3+, and its variation with the pressure. We show that the crystal-field transition energies and their pressure shifts provide a very useful information about the local structural changes in the MnF63- complex and how it changes with the pressure, once the correlations between crystal-field electronic structure and coordination geometry around Mn3+ has been established. Along this work we demonstrate that the equatorial and axial distances decrease from 1.844 to 1.813 Angstrom, and from 2.167 to 2.090 Angstrom, respectively, in the 0-100 kbar range, leading to a partial reduction of the Jahn-Teller distortion.

Author Keywords: Mn3+; MnF63-; Jahn-Teller effect; pressure spectroscopy; layer perovskite

KeyWords Plus: CRYSTAL

DOI: 10.1080/08957950290003094

Author(s): Garcia-Revilla S, Rodriguez F, Valiente R, Pollnau M

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 14    Issue: 3    Pages: 447-460    

Abstract: This work investigates the effect of hydrostatic pressure on the excitation and emission spectra, as well as on the lifetime, of Al2O3:Ti3+ at room temperature. The aim is to establish correlations between the pressure-induced band shifts and the corresponding local structural changes undergone by the TiO6 complex. A blue-shift of 8.52 and 6.86 cm(-1) kbar(-1) was found for the lower (E-1) and upper (E-2) energy components of the excitation band at 17 760 and 20500 cm(-1), respectively, and blue-shifts of 5.93 and 5.40 cm(-1) kbar(-1) for the two overlapping bands of the emission spectrum located at 12 680 and 14210 cm(-1). We explain these results on the basis of a reduction of the TiO6 Jahn-Teller distortion upon increasing the pressure, In contrast, the increase of the overall Stokes shift, which is mainly associated with electron-vibration coupling to the totally symmetric a(1g) vibration, is explained by the increase of the excited-state stabilization energy, S(a1g)homega(a1g), with increasing pressure.

The luminescence lifetime is also found to be pressure dependent, varying from 2.6 mus at ambient conditions to 3.2 mus at 80 kbar. This increase is caused by a diminution of the transition oscillator strength that is related to the odd-vibration assistance mechanism. The softening of the transition mechanism is interpreted in terms of the blue-shift experienced by the O2- –> Ti3+ charge-transfer transition energy upon increasing the pressure.

KeyWords Plus: CHROMIUM PHOTOLUMINESCENCE; LOCAL COMPRESSIBILITIES; STOKES SHIFT; DEPENDENCE; TRANSITIONS; ABSORPTION; ELPASOLITES; SAPPHIRE; DISTANCE; SPECTRA

Author(s): Salley GM, Valiente R, Gudel HU

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 14    Issue: 22    Pages: 5461-5475    

Abstract: Excitation into the Yb3+ F-2(5/2) excited-states leads to visible-by-eye green luminescence spanning the spectral. region from 490 to 790 nm, with a quadratic power dependence. Optical absorption; luminescence; and excitation spectroscopy as well as pulsed measurements on single-crystal SrCl2:Yb(1%), Tb(1%) are used to determine the upconversion (UC) properties of this system. The upconverted luminescence is easily detectable by eye from RT to 100 K, at which point the intensity drops significantly and. a change in colour from green to blue is observed. Pulsed measurements coupled with excitation spectroscopy lead to the unambiguous assignment.of a phonon assisted cooperative sensitization mechanism as the dominant UC process for T > 50 K with VIS/NIR photon ratios on the order of 10(-2)% for a laser power of 56 W mm(-2). Below 50 K, the-dominant UC emission becomes the well-known Yb-Yb cooperative luminescence:. around 500 nm, with a consequential reduction of Tb3+ emission by more than three orders of magnitude from RT to 10 K.

KeyWords Plus: UP-CONVERSION LUMINESCENCE; ENERGY-TRANSFER; GLASSES; YTTERBIUM; CRYSTALS; SYSTEMS; RBMNCL3; YB3+

Author(s): Valiente R, Rodriguez F, Aramburu JA, Moreno M, Barriuso MT, Sousa C, De Graaf C

Source: INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY    Volume: 86    Issue: 2    Pages: 239-244    

Abstract: The variations experienced by the energy E-u(pi) of the e(u)(pi) –> b(1g) (similar tox(2)-y(2)) charge-transfer transition of (C2H5NH3)(2)CdCl4:Cu2+ upon pressure in the 0- to 40-kbar range have been measured at room temperature by means of a sapphire anvil cell. These data reveal that E-u(pi) undergoes a red shift of 1400 cm(-1) on passing from ambient pressure to 40 kbars. To understand this puzzling result theoretical calculations of partial derivativeE(u)(pi)/partial derivativeR(eq) and partial derivativeE(u)(pi)/partial derivativeR(ax) have been performed where R-eq and R-ax mean the equatorial and axial Cu2+-Cl- distances of the elongated CuCl64- complex, respectively. All results indicate that partial derivativeE(u)(pi)/partial derivativeR(eq) and partial derivativeE(u)(pi)/partial derivativeR(ax) for R-eq = 228 pm and R-ax = 297 pm are indeed negative. Moreover ab initio complete active space self-consistent field (CASSCF/CASPT2) and density functional calculations lead to partial derivativeE(u)(pi)/partial derivativeR(ax) values, which are about 10 times smaller than those of partial derivativeE(u)(pi)/partial derivativeR(eq). From the ensemble of experimental and theoretical results, it is concluded that a pressure of 40 kbars gives rise to a decrement of approximate to 25 pm of the axial distance and at the same time to an increase of approximate to 7 pm of the equatorial one. It is stressed that the present study on a diluted Jahn-Teller impurity ties far beyond the current possibilities of X-ray absorption structure techniques. (C) 2002 John Wiley & Sons, Inc.

Author Keywords: CuCl64-; (C2H5NH3)(2)CdCl4 : Cu2+; high-pressure; optical transitions; DFT calculations; ab initio calculations

KeyWords Plus: 2ND-ORDER PERTURBATION-THEORY; CHARGE-TRANSFER TRANSITIONS; MOLECULAR WAVE-FUNCTIONS; ANO BASIS-SETS; HYDROSTATIC-PRESSURE; CRYSTAL-FIELD; DEPENDENCE; APPROXIMATION; EXCITATIONS; COMPLEXES

Author(s): Valiente R, Rodriguez F, Barruso MT, Sousa C, de Graaf C, Aramburu JA, Moreno M

Source: HIGH PRESSURE RESEARCH    Volume: 22    Issue: 2    Special Issue: Sp. Iss. SI    Pages: 475-478    

Abstract: The present work is focused on the effects produced by hydrostatic pressure on Cu2+ (<100ppm) doped (C2H5NH3)(2)CdCl4 single crystal. The energy E-u(pi) of the e(u)(pi) –> b(1)g(similar tox(2) – y(2)) charge-transfer transition of (C2H5NH3)(2)CdCl4: Cu2+ undergoes a red shift of 1400 cm(-1) on passing from ambient pressure to 40 kbar. To understand this fact theoretical calculations of the CuCl64- complex using four different methods have been carried out. Based on these calculations, it is concluded that the experimental red shift undergone by E-u(pi) can only be reasonably explained through a decrease of similar to25 pm in R-ax and an increase of similar to7 pm in R-eq leading to a decrement of the CuCl64- volume. Variations of any metal-halide distances down to 0.1 pm can be well detected through the shifts of the charge-transfer band energy upon pressure.

Author Keywords: high pressure; optical spectroscopy; copper (II); CuCl64-; charge-transfer bands; Jahn-Teller effect

KeyWords Plus: CRYSTAL-FIELD; TRANSITIONS; DEPENDENCE; COMPLEXES

DOI: 10.1080/08957950290014605

Author(s): Valiente R, Wenger OS, Gudel HU

Source: JOURNAL OF CHEMICAL PHYSICS    Volume: 116    Issue: 12    Pages: 5196-5204    

Abstract: Single crystals of CsMnCl3 doped with 0.9% Yb3+ were grown from the melt by the Bridgman technique and studied by means of variable temperature optical spectroscopy. At cryogenic temperatures, near-infrared Yb3+-excitation around 1 mum leads to intense Mn2+ upconversion luminescence in the red spectral region. This very efficient upconversion process is possible because of magnetic Yb3+-Mn2+ exchange interactions, and a new type of upconversion mechanism is found to be active in this system. The upconversion properties of Yb3+:CsMnCl3 are compared to those of Yb3+:RbMnCl3 and Yb3+:CsMnBr3. The upconversion efficiencies at cryogenic temperatures differ by many orders of magnitude. The bridging geometry between Yb3+ and Mn2+ is found to be a key factor for the efficiency of the process. The highest efficiency is observed for the title compound, and this is correlated with the most likely linear Yb3+-Cl–Mn2+ arrangement in this crystal. At 15 K the dominant upconversion mechanism in the title compound involves an energy transfer step. By increasing the temperature to 100 K a new and very efficient mechanism involving a sequence of ground state and excited state absorption steps becomes dominant. (C) 2002 American Institute of Physics.

KeyWords Plus: VISIBLE UP-CONVERSION; COUPLED PAIRS; GD3+ IONS; CSCDBR3; EXCITATION; EPR; CRYSTALS; SPECTRA; RBMNCL3; ENERGY

DOI: 10.1063/1.1446430

Author(s): Wenger OS, Salley GM, Valiente R, Gudel HU

Source: PHYSICAL REVIEW B    Volume: 65    Issue: 21  Article Number: 212108    

Abstract: We present a study of upconversion materials and processes under external hydrostatic pressure. The near-infrared to visible photon upconversion properties of Ti2+-doped NaCl and Ni2+-doped CsCdCl3 at 15 K are studied as a function of external hydrostatic pressure. It is found that in Ti2+:NaCl pressure can be used to switch on an efficient upconversion mechanism, which is inactive at ambient pressure, leading to an order-of-magnitude enhancement of the overall upconversion efficiency of this material. For Ni2+:CsCdCl3 it is demonstrated that upconversion luminescence excitation spectroscopy can be used to study the pressure dependence of excited state absorption transitions. The results demonstrate the ability to tune upconversion properties by altering the local crystal field of active ions, in addition to probing the pressure dependence of excited state absorption transitions via upconversion spectroscopy.

KeyWords Plus: UP-CONVERSION PROPERTIES; CHLORIDE HOST LATTICES; STATE; DEPENDENCE

DOI: 10.1103/PhysRevB.65.212108

Author(s): Wenger OS, Valiente R, Gudel HU

Source: HIGH PRESSURE RESEARCH    Volume: 22    Issue: 1    Special Issue: Sp. Iss. SI    Pages: 57-62    

Abstract: The near-infrared to visible upconversion properties of Ni2+ doped CsCdCl3 are studied as a function of hydrostatic pressure. At 15 K and ambient pressure, near-infrared excitation of Ni2+ around 810nm leads to upconversion luminescence centered around 610nm. Due to the increase of the ligand field strength with increasing pressure, the emission band moves to higher energies with a rate of 19 cm(-1)/kbar, and at 46 Bar it is centered around 585nm. Thus the upconversion luminescence undergoes a color change from red to yellow in the I bar-46kbar pressure range. The 15 K pressure-dependent upconversion excitation spectra reveal a slight red-shift of -4cm(-1)/kbar of the upconversion relevant T-3(2g) –> T-1(2g) excited state absorption (ESA) transition, and this is related to a reduction of the Racah B and C parameters, caused by an increasing degree of covalency in the Ni2+-Cl- interaction towards higher pressures. Thus, with upconversion luminescence excitation spectroscopy we can obtain information about the pressure-dependence of an ESA transition. The pressure-induced intensity redistributions within this excitation spectrum are indicative of an increasing spectral overlap of T-3(2g) –> T-1(2g) ESA with (3)A(2g) –> T-3(1g) (F-3) ground state absorption, potentially leading to more efficient upconversion at higher pressures than at ambient pressure.

Author Keywords: high pressure; upconversion; luminescence; optical spectroscopy; Ni2+

KeyWords Plus: CHLORIDE; LATTICES; SPECTROSCOPY

DOI: 10.1080/08957950290007207

Author(s): Wenger OS, Valiente R, Gudel HU

Source: JOURNAL OF CHEMICAL PHYSICS    Volume: 115    Issue: 8    Pages: 3819-3826    

Abstract: The T-4(2g)–> (4)A(2g) luminescence of a 4.1% Cr3+ doped Cs2NaScCl6 crystal is studied as a function of hydrostatic pressure at room temperature and 15 K. The vibrational fine structure observed in the low-temperature variable pressure emission spectra is analyzed with a two configurational coordinate approach, involving the totally symmetric a(1g) and the e(g) Jahn-Teller normal coordinate. Increasing hydrostatic pressure is found to reduce the tetragonal distortion of the CrCl63- unit in the electronic T-4(2g) state resulting from the Jahn-Teller effect. Additionally, pressure impedes expansion along the a(1g) coordinate of the CrCl63- complex upon (4)A(2g)–> T-4(2g) photo-excitation, and thus has a greater influence on the T-4(2g) excited state than on the (4)A(2g) ground state. The absolute Cr3+-Cl- average distance reduction upon increasing pressure is estimated using a simple point charge model. (C) 2001 American Institute of Physics.

KeyWords Plus: NEAR-INFRARED LUMINESCENCE; CHROMIUM PHOTOLUMINESCENCE; TEMPERATURE-DEPENDENCE; ABSORPTION-SPECTRA; RUBY; SPECTROSCOPY; K2NAGAF6-CR3+; ELPASOLITES; CALIBRATION; RELAXATION

Author(s): Wenger OS, Valiente R, Gudel HU

Source: PHYSICAL REVIEW B    Volume: 64    Issue: 23  Article Number:

Abstract: Single crystals of Ni2+-doped Rb2CdCl4 and Rb2MnCl4 are studied by polarized optical absorption and luminescence spectroscopy at 15 K. In Ni2+:Rb2MnCl4, Ni2+-Mn2+ exchange interactions, which occur exclusively within the perovskite layers of this material, dramatically affect the absorption, luminescence, and up-conversion properties of Ni2+ for light polarized within the layers (sigma): The Ni2+ (3)A(2g)–>E-1(g) and (1)A(1g) ground-state-absorption (GSA) transitions are each enhanced by about an order of magnitude when compared to Ni2+:Rb2CdCl4. Similarly, in Ni2+:Rb2MnCl4 the T-3(2g)–>T-1(2g) excited-state absorption (ESA) as well as its reverse luminescence transition experience an intensity enhancement in their electronic origin region in sigma polarization. One- and two-color up-conversion-excitation experiments show that, due to the in-plane exchange enhancement of the up-conversion-relevant GSA and ESA transitions in Ni2+Rb2MnCl4, the near-infrared(NIR) to-visible (VIS) up-conversion process at 15 K in this material is up to a factor of 55 more efficient for sigma than for pi -polarized excitation, and also one to two orders of magnitude more efficient than in Ni2+:Rb2CdCl4 at the same temperature. The structural conditions for efficient Ni2+-Mn2+ exchange interactions are discussed. The NIR-to-VIS up-conversion efficiency can be further enhanced by an energy-transfer step from Ni2+ T-1(2g) to Mn2+ T-4(1g), however, this occurs very inefficiently in Ni2+:Rb2MnCl4.

KeyWords Plus: EXCITED-STATE LUMINESCENCE; CHLORIDE HOST LATTICES; SUPEREXCHANGE INTERACTION; NONRADIATIVE PROCESSES; ELECTRONIC EXCITATION; NICKEL IONS; MANGANESE; DYNAMICS; SPECTRA; COMPLEXES

DOI: 10.1103/PhysRevLett.64.235116

Author(s): Gutierrez RE, Rodriguez F, Moreno M, Alcala R

Source: RADIATION EFFECTS AND DEFECTS IN SOLIDS    Volume: 154    Issue: 3-4    Pages: 287-293    Published: 2001  

Abstract: This work investigates the surprising disappearance of the Mn2+ green photoluminescence (PL) observed at room temperature on passing from CaF2 to SrF2 along the Ca1 – xSrx:Mn2+ fluorite series. The aim is to understand the microscopic origin of the excited-state relaxation phenomena leading to radiationless processes in these crystals. High-pressure experiments performed on Ca0.25Sr0.75F2:Mn2+ show that PL can gradually recover by application of pressure. The increase of intensity and lifetime with pressure is explained by a reduction of the fluorite lattice parameter, a. The variation of Mn2+ PL lifetime with pressure and x is described through the same equation by renormalising these parameters to a.

Author Keywords: high-pressure spectroscopy; photoluminescence; radiationless process; Mn2+; fluorite series

KeyWords Plus: MN2+ IONS

Author(s): Salley GM, Valiente R, Guedel HU

Source: JOURNAL OF LUMINESCENCE    Volume: 94    Pages: 305-309    

Abstract: Tb3+ (D-5(4) –> F-7(3)) luminescence has been observed in single crystals of SrCl2 : Tb3+ (1%) : Yb3+ (1%) and Cs3Tb2Br9 : Yb3+ (1%) under excitation energies in the region of Yb3+ absorption. Previous reports of similar systems have postulated two possible upconversion mechanisms: (1) cooperative sensitization and (2) a sequence of two cooperative absorption steps, or GSA/ESA. We present results of emission. excitation and kinetic measurements at low and high temperatures. These measurements allow assignment of the mechanism responsible for upconversion in these systems. For temperatures greater than T = 100 K process (1) is the dominant mechanism, in agreement with previous assignments. However, for T < 100 K in Cs3Tb2Br9 : Yb3+ (1%), the upconversion occurs through a GSA/ESA sequence, which is shown here in a clear manner for the first time. The efficiency of the Yb3+-Tb3+ upconversion process for SrCl2 : Tb3+ (1%) : Yb3+ (1%), at room temperature, under 2.4( 10)W-4/cm(2), is on the order of 10(-4) and decreases with decreasing temperature by four orders of magnitude. (C) 2001 Elsevier Science B.V. All rights reserved.

Author Keywords: upconversion; Tb^3-: Yb³⁺

KeyWords Plus: UP-CONVERSION; YB3+

Author(s): Valiente R, Rodriguez F, Moreno M, Lezama L

Source: VIBRONIC INTERACTIONS: JAHN-TELLER EFFECT IN CRYSTALS AND MOLECULES   Book Series: NATO SCIENCE SERIES, SERIES II: MATHEMATICS, PHYSICS AND CHEMISTRY    Volume: 39    Pages: 221-228    

Abstract: This work investigates the local structure around Cu2+, in the two-dimensional perovskite layers (CnH2n+1NH3)(2)CdCl4 (n = 1, 3) doped with Cu2+. The interplay between the Jahn-Teller effect and the host lattice compressive strain is analyzed through EPR spectroscopy as a function of the temperature in both powder and single crystals doped with different Cu2+ concentrations. The high tendency Of Cu2+ to form aggregates in these layered systems makes it difficult to study the isolated CuCl64- units. Besides, it is shown that Cu2+ replaces Cd2+ by distorting the compressed D-4h site symmetry to an elongated rhombic CuCl64- geometry. The long Cu-Cl axis is randomly distributed between either of the two equivalent Cd-Cl directions within the layer. The temperature dependence of the principal gyromagnetic values and the A(II) hyperfine constant reveal the existence of a dynamic Jahn-Teller effect related, not to three potential wells as it is usually found in octahedral sites, but to two potential wells. This behavior is explained by the energy increase of the potential well associated with the out-of-layer Cu-Cl elongation, due to site anisotropy. The influence of the D-2h symmetry upon the isotropic hyperfine constant is briefly discussed.

Author Keywords: dynamic and static Jahn-Teller effect; EPR spectroscopy; CU2+

KeyWords Plus: CUCL4(NH3)22 CENTER; SPECTRA

Author(s): Rodriguez F, Hanfland M, Itie JP, Polian A

Source: FRONTIERS OF HIGH PRESSURE RESEARCH II: APPLICATION OF HIGH PRESSURE TO LOW-DIMENSIONAL NOVEL ELECTRONIC MATERIALS   Book Series: NATO SCIENCE SERIES, SERIES II: MATHEMATICS, PHYSICS AND CHEMISTRY    Volume: 48    Pages: 143-153    Published: 2001  

Abstract: This work investigates the optical spectra of Cu(II) layered perovskites under pressure. The aim is to establish correlations between the metal-to-ligand charge transfer spectra and the local structure around the Cu(II), For this purpose, X-ray diffraction (XRD) and X-ray absorption (XAS) experiments on the title compounds were performed as a function of pressure in the 0-100 kbar range. The pressure redshift experienced by the first charge transfer band correlates with a reduction of the Janh-Teller (JT) distortion of the hexachloride Cu(II) complex. Interestingly, the application of pressure to these layer compounds induces octahedron tilts rather than a reduction of the in-plane antiferrodistortive structure related to the cooperative JT structure. This reflects the stiffness of the JT Cu complex whose local bulk modulus is an order of magnitude greater than the crystal bulk modulus.

Author Keywords: pressure spectroscopy; XAS; XRD; Jahn-Teller systems; Cu(Il)-layered perovskites

KeyWords Plus: CL–>CU2+ CHARGE-TRANSFER; HYDROSTATIC-PRESSURE; SPECTRA; DIFFRACTION; CU2+; ABSORPTION; SYSTEMS; SERIES; K2ZNF4

Author(s): Rodriguez F, Davies G, Lightowlers EC

Source: PHYSICAL REVIEW B    Volume: 62    Issue: 10    Pages: 6180-6191    

Abstract: We report on a new Li-related photoluminescence center with zero-phonon line at 879.3 meV. The center is created at 550-600 degrees C in the final stages of annealing out radiation-induced point defects in float-zone silicon. Isotope and chemical correlation data establish that the center contains Li and C atoms. The isotope shift from Li-6 to Li-7, Delta E = E(Li-7) – E(Li-6) = 0.18 meV, is similar, per Li atom, to that observed for other Li-related centers in silicon. Uniaxial stress measurements establish the symmetry as monoclinic I, with only small departures from trigonal symmetry. A simple method for the transition is introduced to fit simultaneously the energies, polarizations, and relative intensities of the stress-split components. The transition’s dipole is shown to be close to a bonding direction in the plane perpendicular to the characteristic [110] axis of the monoclinic I center. The vibronic sideband is produced by coupling to modes of 16, 31, and 36 meV, with a Huang-Rhys factor S = 1.1. This value can be predicted simply from the uniaxial-stress data. The temperature dependence of the zero-phonon line can be fitted precisely using the spectrum of coupled phonons derived from the vibronic band shape, plus the approximation that the differences in frequencies of the phonons in the ground and excited electronic states are proportional to the phonon frequency. The luminescence from the center is reversibly quenched with increasing temperature, with an activation energy E-a = 32+/-5 meV, however, we show that this result does not arise from the excited state containing a shallow particle. Although the center is created at 600 degrees C, it is rapidly destroyed at room temperature through passivation by the capture of one mobile Li atom.

KeyWords Plus: UNIAXIAL-STRESS; LUMINESCENCE-CENTERS; CRYSTALLINE SILICON; CARBON; LITHIUM; DEFECT; SPECTRUM; ABSORPTION; COMPLEXES; BAND

Reprint Address: Rodriguez, F (reprint author), Univ Cantabria, Fac Ciencias, DCITIMAC, E-39005 Santander, Spain

Author(s): Rodriguez F, Hernandez D, Garcia-Jaca J, Ehrenberg H, Weitzel H

Source: PHYSICAL REVIEW B    Volume: 61    Issue: 24    Pages: 16497-16501    

Abstract: The aim of this work is to investigate the origin of the piezochromism and thermochromism exhibited by the copper oxide CuMoO4. These optical phenomena are associated with structural phase transition (PT) from the triclinic alpha (green) modification to the gamma (brownish-red) modification. The variation of the optical-absorption spectrum with pressure and temperature indicates that the piezochromic and thermochromic transitions can be leached from ambient conditions either by applying pressure at 2.5 kbar or by cooling at T = 200 K. We show that the change of color at the alpha –> gamma PT is due to the broadening of the first O2- –> Cu2+ charge-transfer band, and the disappearance of an intense peak at 1.49 eV, related to the presence of pyramidal CuO5 complexes in alpha-CuMoO4. The measured oscillator strength suggests that this peak corresponds to the e–>b(1) crystal-field transition within CuO5 rather than to an O2—> Cu2+ charge-transfer band. The correlation between optical and structural properties performed in this work confirms this interpretation, and also explains the strong dichroism exhibited by the crystal in the high-pressure gamma- CuMoO4 modification.

KeyWords Plus: CRYSTAL; PHASE

Author(s): Rodriguez F, Valiente R, Espeso JI, Benfatto M, Pascarelli S

Source: HIGH PRESSURE RESEARCH    Volume: 18    Issue: 1-6    Pages: 165-171    

Abstract: This work investigates the X-ray absorption spectra of the Cu K-edge in the layered compounds (CnH2n+1NH3)(2)CuCl4 (n=1-3) and their variation with pressure. The XANES spectra show a three peaked structure around 8.98 KeV at zero pressure which is characteristic of Cu2+ complexes in D-4h symmetry. Application of hydrostatic pressure induces a progressive reduction of the energy separation between the second and third peaks that has been interpreted in terms of a progressive disappearance of the antiferrodistortive structure exhibited by the Cu2+ complexes in this compound series. The results are compared with recent pressure experiments on Raman and Charge Transfer spectroscopy as well as with Multiple Scattering calculations performed on these layered systems.

Author Keywords: XANES; pressure spectroscopy; Jahn-Teller; CuCl6 complex; Cu2+-layers; Multiple Scattering

KeyWords Plus: CL–>CU2+ CHARGE-TRANSFER; SYSTEMS; SPECTRA

Author(s): Valiente R, Rodriguez F

Source: RADIATION EFFECTS AND DEFECTS IN SOLIDS    Volume: 149    Issue: 1-4    Pages: 81-87

 Abstract: The aim of this work is to investigate the Charge Transfer (CT) spectra of CuCl42- and CuBr42- complexes formed In Cu2+-doped A(2)MX(4) crystals and their dependence with chemical pressure when Cu2+ replaces the divalent cation at the MCl42- tetrahedral site. A salient feature of this study is the weak sensitivity exhibited by the CT bands to structural changes of the complex. A similar phenomenon has also been observed for pure chlorocuprates. The analysis carried out on the available family of chlorocuprates indicates that the CT band-shift induced by structural distortions in CuCl42- is smaller than that of the corresponding Crystal Field (CF) bands. We present a perturbative model for explaining the weak sensitivity of the CT bands in comparison to the CF ones. The estimates of the model accounts for the spectroscopic results observed along the A(2)MCl(4):Cu2+ series.

Author Keywords: tetrahedral complex; charge transfer transition; Jahn-Teller distortion; Cucl(4)(2-); CuBr₄^2-

KeyWords Plus: CUCL42 ANIONS; CUBR42 COMPLEXES; TRANSFER SPECTRA; TETRACHLOROCUPRATE(II); CRYSTALS; TMA2MNBR4; GEOMETRY

Author(s): Valiente R, Rodriguez F

Source: JOURNAL OF PHYSICS-CONDENSED MATTER     Volume: 10    Issue: 42    Pages: 9525-9534

 Abstract: This work investigates the charge-transfer spectra of CuCl42- and CuBr42- complexes formed in anisotropic A(2)MX(4) (X = Cl, Br) crystals. Attention is paid to the variations of the charge-transfer transition energies induced by chemical pressure effects when substituting Mn –> Zn –> Cd –> Hg along the crystal series. A salient feature of this study is the weak sensitivity of these bands to structural changes of the complex in comparison to the sensitivity of the corresponding crystal field (d-d) bands. The knowledge of these structural-induced shifts is important since they are responsible to a great extent for the thermo- and piezochromic properties exhibited by some compounds containing CuX42- units as chromophores. We present a tentative model based on a perturbed tetrahedral CuX42- complex for explaining the weak sensitivity exhibited by the charge-transfer transitions in the title compounds. Furthermore the estimates of this model can also explain the big difference between the energy shift of d-d and charge-transfer transitions due to structural changes of the CuCl42- complex in pure chlorocuprates.

Keywords: TRANSFER TRANSITIONS; CUBR42 COMPLEXES; CUCL42 ANIONS; SQUARE-PLANAR; TETRACHLOROCUPRATE(II); CRYSTALS; DISTORTION; TMA2MNBR4; PHASES

Author(s): Valiente R, Rodriguez F

Source: JOURNAL OF PHYSICS-CONDENSED MATTER     Volume: 10    Issue: 42    Pages: 9525-9534

 Abstract: This work investigates the charge-transfer spectra of CuCl42- and CuBr42- complexes formed in anisotropic A(2)MX(4) (X = Cl, Br) crystals. Attention is paid to the variations of the charge-transfer transition energies induced by chemical pressure effects when substituting Mn –> Zn –> Cd –> Hg along the crystal series. A salient feature of this study is the weak sensitivity of these bands to structural changes of the complex in comparison to the sensitivity of the corresponding crystal field (d-d) bands. The knowledge of these structural-induced shifts is important since they are responsible to a great extent for the thermo- and piezochromic properties exhibited by some compounds containing CuX42- units as chromophores. We present a tentative model based on a perturbed tetrahedral CuX42- complex for explaining the weak sensitivity exhibited by the charge-transfer transitions in the title compounds. Furthermore the estimates of this model can also explain the big difference between the energy shift of d-d and charge-transfer transitions due to structural changes of the CuCl42- complex in pure chlorocuprates.

Keywords: TRANSFER TRANSITIONS; CUBR42 COMPLEXES; CUCL42 ANIONS; SQUARE-PLANAR; TETRACHLOROCUPRATE(II); CRYSTALS; DISTORTION; TMA2MNBR4; PHASES

Author(s): Moral BA, Rodriguez F

Source: JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS     Volume: 58    Issue: 9    Pages: 1487-1490

 Abstract: This paper investigates the variation of the charge transfer (CT) spectra of the Cu2+ doped (EtNH3)(2)CdCl4 (Et = C2H5) layer perovskite under hydrostatic pressure. The aim is to explain the strong redshift (-4000 cm(-1)) undergone by the first CT band of the formed CuCl64- complex on passing from (EtNH3)(2)CdCl4: Cu2+ to the isomorphous (EtNH3)(2)MnCl4: Cu2+ crystal, which is accompanied by a change of color from light yellow to deep red. The pressure experiments show that the first Cl- –> Cu2+ CT band experience an abrupt shift of -1200 cm(-1) between 23 and 26 kbar. The comparison of these results with those previously obtained in the pure (EtNH3)(2)CuCl4 suggests that this shift is associated with a local change around the Cu2+ from an elongated octahedron geometry (P = 1 atm) to a more compressed situation similar to that found for the Mn crystal. (C) 1997 Elsevier Science Ltd. All rights reserved.

Keywords: CHLORIDE; TRANSITIONS; DISTORTION; ABSORPTION; CRYSTALS; K2ZNF4; ION

Author(s): Moral BA, Rodriguez F, Valiente R, Valiente R, Lezama LM, Rodriguez F, Moreno MU

Source: PROCEEDINGS OF THE 13TH INTERNATIONAL CONFERENCE ON DEFECTS IN INSULATING MATERIALS – ICDIM 96 MATERIALS SCIENCE FORUM    Volume: 239    Issue:     Pages: 729-732

 Abstract:

The bidimensional (CH3NH3)(2)CdCl4 lattice containing isolated Cu2+ ions has been explored through the charge transfer and EPR spectra obtained in the 10 – 300 K temperature range. Both spectra are consistent with the existence of two different sites for Cu2+, but where the same D-2h CUCl64- center is formed. The longest and the intermediate Cu2+-Cl- bonds of the complex are alternated in the layer (a,b) plane, while the shortest bond is the c direction perpendicular to the layer. The temperature dependence of the EPR spectra demonstrates the existence of dynamic features associated with two (and not three) equivalent wells on a given site. All these features are consistent with the existence of an additional strain term (reflecting the bidimensional host lattice), as well as the usual Jahn-Teller term in cubic symmetry.

The importance of this fact for a possible transition to a D-4h compressed geometry under pressure is discussed. The EPR spectrum of (CH3NH3)(2)CUCl4 in the (a,b) plane is isotropic due to the exchange interaction between the two Cu2+ sites.

Keywords: Jahn-Teller effect; two equivalents wells; charge transfer bands

Author(s):Moral BA, Rodriguez F, Valiente R, Moreno M, Gudel HU

Source: ZEITSCHRIFT FUR PHYSIKALISCHE CHEMIE-INTERNATIONAL JOURNAL OF RESEARCH IN PHYSICAL CHEMISTRY & CHEMICAL PHYSICS     Volume: 201    Issue:     Pages: 151-158

 Abstract: This paper deals with the effects of pressure on the charge-transfer spectra of CuCl64- complexes formed in Cu2+ doped (C2H5NH3)(2)MCl4 (M = Cd, Mn). A pressure-induced redshift is observed for the first charge-transfer band in both crystals. While the shift is continuous for the Mn crystal at a rate of -40 cm(-1)/kbar in the 0-60 kbar range, it experiences an abrupt jump of -1400 cm(-1) around 26 kbar for the Cd crystal. Such a discontinuous behaviour is interpreted in terms of a structural change in the CuCl64- coordination geometry from an axially elongated octahedron to a more compressed geometry. The present results are compared with those reported for the pure (C2H5NH3)(2)CuCl4 crystal.

Keywords: charge-transfer spectroscopy; hydrostatic pressure; CuCl₆^4--complex;; (C₂H₅NH₃)₂MnCl₄  (C₂H₅NH₃)₂CdCl₄  

Author(s): Valiente R, Rodriguez F

Source: ZEITSCHRIFT FUR PHYSIKALISCHE CHEMIE-INTERNATIONAL JOURNAL OF RESEARCH IN PHYSICAL CHEMISTRY & CHEMICAL PHYSICS     Volume: 201    Issue:     Pages: 159-164

 Abstract:

This work investigates the polarized Charge-transfer spectra associated with Cu2+ impurities in [(CH3)(4)N]HgBr3:Cu2+ in the 10-300 K temperature range. Attention is paid for correlating the optical spectra with the local structure around Cu2+. The spectra are compared with those obtained in the Cu2+ doped [(CH3)(4)N]CdBr3 and [(CH3)(4)N]MnBr3, and [(CH3)(4)N](2)HgBr4 and [(CH3)(4)N](2)CdBr4 crystals, where the Cu2+ ions form CuBr64- complexes of nearly D-4h symmetry and tetrahedral distorted CuBr42–complexes of D-2d symmetry, respectively. The presence of a strongly-polarized absorption band at 15900 cm(-1) in [(CH3)(4)N]HgBr3:Cu2+ is interpreted in terms of a highly distorted CuBr42- complex. The enhancement of dichroism exhibited by this crystal with decreasing temperature is noteworthy. The analysis of the spectra reveals that this unusual thermal behaviour is mainly associated with orientational motions of the Cu2+ complex rather than with structural changes of the complex.

Keywords: charge-transfer spectra; Cu2+ impurities; copper bromide complexes 

Author(s): Núñez P, Elias C, Fuentes J, Solans X, Tressaud A, deLucas MCM, Rodríguez F

Source: JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS    Volume:     Issue: 22    Pages: 4335-4340

 Abstract: The syntheses and crystal structures of two new fluoromanganese(III) complexes are reported; [MnF3(H2O)(2, 2′-bipy)] 1 and 4, 4′-bipyH(2)[MnF4(H2O)(2)].2H(2)O 2, where 2, 2′-bipy and 4, 4′-bipy are 2, 2′-bipyridyl and 4, 4′-bipyridyl, respectively. Compound 1: monoclinic, space group P2(1)/n, a = 1973.7(4), b = 749.0(2), c = 903.1(3) pm, beta = 95.22(3)degrees, Z = 4, R1 = 0.051. Compound 2: monoclinic, space group P2(1), a = 516.4(2), b = 1851.9(4), c = 986.3(3) pm, beta = 99.07(2)degrees, Z = 2, R1 = 0.028. The manganese co-ordination environment was found to be octahedral in both compounds, but strongly distorted by the Jahn-Teller effect as a result of the high-spin d(4) configuration of Mn3+. A very extensive intermolecular hydrogen-bond framework is present in both compounds. For compound 1 the octahedra are linked through hydrogen bridges resulting in octahedral manganese chains. For compound 2, the octahedra [MnF4(H2O)(2)](-) are associated via hydrogen bonds into chains, which in turn are connected by interchain hydrogen bridges. The polarized optical spectra of single crystals are presented and explained in terms of intraconfigurational d(4) transitions split by ligand fields of nearly C-s and D-4h symmetries for compounds 1 and 2, respectively. The results are compared with those available for other Mn-III fluorides.

Keywords: MAGNETIC-STRUCTURES; CRYSTAL-STRUCTURE; ALKALI-METAL; MANGANESE(III); FLUORIDES; COMPOUND

Author(s): Valiente R, Rodriguez

Source: JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS     Volume: 57    Issue:     Pages: 571-587

 Abstract: The polarized optical absorption spectra of the 2-D A(2)Cd(1-x)Mn(x)Cl(4) (x = 0-1; A = CH3NH3) crystals doped with Cu2+ investigated. The analysis of both the charge transfer and crystal field spectra indicates that the copper impurities form CuCl64- complexes with an elongated D-2h (nearly D-4h) symmetry. A salient feature is the enhancement of charge transfer band intensity as well as the presence of new intense bands at 21000 and 25000 cm(-1) observed on passing from x = 0 to x = 1 along the series. These bands are associated with Mn-Cu aggregates whose superexchange pathways involve the short (equatorial) Cu-Cl bond (25000 cm(-1)) and the long (axial) Cu-Cl bond (21000 cm(-1)) of the CuCl64- complex. A noteworthy fact of these exchange coupled Mn-Cu systems is that the transition energy of the first Mn2+ excitations are resonant with the Cl- –> Cu2+ charge transfer transitions. This effect seems to play a fundamental role in the enhancement of intensity. The temperature dependence of the oscillator strength allowed us to estimate a ground state exchange constant J approximate to 70 cm(-1) for the Mn-Cu pair. The results are compared with those obtained in Mn-Cu aggregates in fluorides.

Keywords: CuCl64-; exchange coupled Mn-Cu systems; charge transfer spectra; (RNH(3))(2)Cd1-xMnxCl4 crystals; spectroscopic and structural correlations

Author(s): deLucas MCM, Moreno M, Rodríguez F, Baranov PG

Source: JOURNAL OF PHYSICS-CONDENSED MATTER     Volume: 8   Issue:  14  Pages: 2457-2465

 Abstract: Through this work it is shown Chat the luminescence observed in BaF2:Mn2+ samples is related to the formation of BaMnF4 microcrystalline precipitates. This conclusion is supported by the experimental emission and excitation spectra as well as by the temperature dependence displayed by the emission intensity. Also the EPR spectrum of most doped samples clearly shows the presence of a signal (superimposed on that coming from isolated Mn2+ ions) whose temperature dependence is similar to that displayed by BaMnF4 crystals. The absence of luminescence associated with isolated Mn2+ ions in the BaF2 host lattice is discussed. It is pointed out that luminescence quenching is probably related to a multiphonon non-radiative process rather than to a T-4(1) (G) excited state close to the bottom of the BaF2 conduction band. Although luminescence quenching appears for a Dexter-Klick-Russell parameter A probably close to 0.1, this is qualitatively related to a high radiative lifetime (tau(R) similar or equal to 100 ms). The analysis made also shows that firstly Lambda increases by a factor of about 1.7 on going from Mn2+-doped fluoroperovskites to Mn2+ in fluoride-type crystals, and secondly Lambda increases on going from CaF2 : Mn2+ to SrF2 : Mn-2. Both variations mainly reflect a parallel increase in the Stokes shift.

Keywords: LOCAL INSTABILITY; LATTICES; PHOTOLUMINESCENCE; RELAXATION; ABSORPTION; CRYSTALS; DISTANCE; FLUORIDE; IMPURITY; SPECTRUM

Author(s): Baticle B, Rodriguez F, Valiente R

Source: RADIATION EFFECTS AND DEFECTS IN SOLIDS       Volume: 135  Issue:  1-4  Pages: 587-592

 Abstract:

This work investigates the charge transfer spectra of CuCl64- complexes formed in the (RNH(3))(2)CdxMn1-xCl4 crystal series. For x = 1, it is demonstrated that Cu complexes display an elongated D-4h symmetry, forming an antiferrodistortive type structure which is similar to that found in the pure copper crystals. The presence of Mn induces an important enhancement of the charge transfer bands as well as significant redshifts. The results are interpreted in terms of distortions of the CuCl64- geometry which changes from D-4h elongated (x = 1) to nearly D-4h compressed (x = 0). The transition energy, polarization and assignment of the charge transfer bands are analysed.

Keywords: CuCl64- comples; D-4h elongated and compressed symmetries; charge transfer spectra; (RNH(3))(2)CdxMn1-xCl4 crystals

Author(s): MORAL BA, RODRIGUEZ F

Source: REVIEW OF SCIENTIFIC INSTRUMENTS      Volume: 66  Issue:  11  Pages: 5178-5182

 Abstract: This article describes a new double beam spectrophotometer specially conceived for optical absorption measurements on low-absorbing microsamples. The available long-working distance makes this apparatus attractive for use on samples placed in special environments such as heating stages, biological cells, and particularly hydrostatic pressure cells. Its performance has been tested in optical absorption measurements for different Mn2+ and Cu2+ complexes. We applied the instrument for investigating the electronic spectrum of Cu2+ doped (CH3CH2NH3)(2)CdCl4 crystals under hydrostatic pressure using a Sapphire anvil cell. A salient feature of this work is the enormous redshift (1400 cm(-1)) experienced by the first Cl- –> Cu2+ charge transfer band at 26 kbar. We briefly comment on the origin of this shift. (C) 1995 American Institute of Physics.

Keywords: DIAMOND-ANVIL CELL; PHASE-TRANSITIONS; CRYSTALS; SYSTEM

Author(s): VALIENTE R, DELUCAS MCM, RODRIGUEZ F

Source: JOURNAL OF PHYSICS-CONDENSED MATTER      Volume: 7 Issue:  20  Pages: 3881-3894

Abstract: The charge-transfer electronic structure of CuBr42- in [N(CH3)(4)]2CdBr4 crystals is investigated through polarized optical absorption spectroscopy. The transition energy and the band polarization are explained in terms of the Jahn-Teller distortions of D-2d symmetry of the CuBr42- complex. Some bands are split in the low-temperature spectra by the spinorbit interaction of the Br ligands. The results are compared with those available for other CuBr42- systems. We also investigate the dynamics of the inorganic CuBr42- units in the 9.5-300 K temperature range through the intensity of the charge-transfer bands. Analogously to the CdBr42- tetrahedra in the pure crystal, CuB42- experiences reorientational motions upon varying the temperature which are correlated with the temperature dependence of the monoclinic beta parameter below the Pmcn –> P2(1)/c phase transition temperature T-cl = 272 K. An important finding of the present work is the observation of anomalies in the CuBr42- dynamics which are associated with the existence of a new first-order phase transition at T-c2 = 20 K with a thermal hysteresis Delta T = 10 K. This new phase transition, which had not previously been detected in the [(CH3)(4)N](2)MBr(4) (M = Zn, Mn, Co or Cd) series, would correspond to the monoclinic P2(1)/c to orthorhombic P2(1)2(1)2(1) transition, confirming the predictions of the universal p-T phase diagrams of the title compounds.

Keywords: TRANSFER BANDS; CUCL42 ANIONS; TETRACHLOROCUPRATE(II); DISTORTION; CONNECTION; NH4CL-CU2+; TMA2MNBR4; COMPLEXES; CO; CU

Author(s): DeLucas MCM, Dance JM, Rodriguez F, Tressaud A, Moreno M, Grannec J

Source: RADIATION EFFECTS AND DEFECTS IN SOLIDS      Volume: 135 Issue: 1-4  Pages: 517-520

Abstract:

New elpasolites like A2KMF6 doped with Cr³⁺ have been investigated by means of optical and EPR techniques. It is shown that though the structural phase transition undergone by Rb2KGaF6 at T = 129 K is much stronger than that corresponding to Rb2KInF6, the emission band at 10 K of the latter is much broader than that for the former. This inhomogeneous broadening is correlated with the increase experienced by the EPR bandwidth below 150 K reflecting only local changes around Cr3+. An analysis of zero phonon lines through the fluoroelpasolite series reveals that they are very sensitive to changes of the Cr3+-F- distance, R. It is shown that changes Delta R(o) in the perfect lattice induce changes on R given by Delta R = fR(o) where fis only 0.15. At variance with the findings for MnF64-, the present results indicate that an increase of R tends to decrease the Stokes shift. The origin of this fact is discussed. Finally, the present results indicate that R increases (about 0.2 pm) in the phase transition of Rb2KGaF6 although the lattice volume decreases.

Keywords: fluoroelpasolites doped with Cr3+; luminescence of Cr3+; EPR of Cr3+; phase transitions; influence of the Cr-F distance

Author(s): DeLucas MCM, Rodriguez F, Prieto C, Verdaguer M, Moreno M, Gudel HU

Source: RADIATION EFFECTS AND DEFECTS IN SOLIDS      Volume: 135 Issue: 1-4  Pages: 593-598

Abstract: The optical properties of ABCl(3):Mn2+ crystals are investigated in the 300-10 K temperature range. The variation of the peak energy and the Stokes shift along the series are explained in terms of slight differences in the Mn-CI distance. The local structure around the Mn is determined by correlating optical spectroscopy and EXAFS techniques. Interestingly, the thermal shift of the (6)A(1g) –> T-4(1g) excitation band is much smaller than that experienced by the corresponding emission band. This behaviour is explained by the phonon assisted mechanism involve in these transitions. The influence of the structural phase transition of the CsCaCl3:Mn2+ at T-C = 95 K upon the thermal band shift is also analysed.

Keywords: excitation and luminescence; ABCl(3):Mn2+ perovskites; MnCl64- complex; thermal shift; local structure around Mn2+; influence of bond distances

Author(s): DELUCAS MCM, RODRIGUEZ F, MORENO M

Source: JOURNAL OF PHYSICS-CONDENSED MATTER      Volume: 7 Issue: 38  Pages: 7535-7548 

Abstract: The thermal shifts undergone by the first moment of the (6)A(1g)(S) –> T-4(1g)(G) excitation band and the associated emission band of Mn2+-doped ABF(3) perovskites are investigated in the 9-300 K temperature range. It is found that these shifts are similar for the whole series and have average values of +150 and +450 cm(-1) for excitation and emission, respectively. Both the sign and the magnitude of these different thermal shifts are explained in terms of (i) the phonon assistance mechanism required to gain intensity of the parity-forbidden transitions, (ii) the quadratic electron-phonon coupling and (iii) thermal expansion effects. To achieve this analysis a previous discussion upon the nature of the vibrational modes seen in the optical spectra is carried out. It is stressed that the impurity vibrational mode displaying HBAR omega(g) = 570 cm(-1) in the emission spectrum of KMgF3:Mn2+ exhibits a value of 540 cm(-1) in the corresponding excitation spectrum. This situation, which is also found for other modes seen in the optical spectra of KMgF3:Mn2+, indicates that the mode (though associated with the LO(3) branch of KMgF3) is not a pure mode of the lattice but displays a kind of resonant character. As a salient feature the calculated thermal shifts are based on the experimental shifts experienced by the frequencies of the optical and acoustic modes on going from the ground (6)A(1g) to the excited T-4(1g) state of MnF64-. At variance with findings for the R lines in Cr3+ and V2+, it is clearly demonstrated that the explicit and implicit contributions to the thermal shift of the zero-phonon line in MnF64- are similar and both induce red shifts upon heating. Moreover the present analysis reveals that the explicit contribution to the thermal shift undergone by the zero-phonon line of KMgF3:Mn2+ is mainly dominated by the odd-parity low-energy modes. The calculated thermal shifts reproduce reasonably well the experimental data.

Keywords: TEMPERATURE-DEPENDENCE; 2-EXCITON BANDS; RBMNF3; KMGF3; PHOTOLUMINESCENCE; TRANSITIONS; KZNF3; K2NAGAF6-CR3+; ABSORPTION; SPECTRUM

Author(s): DELUCAS MCM, RODRIGUEZ F, MORENO M

Source: JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS      Volume: 56 Issue: 7  Pages: 995-1001 

Abstract: This work reports the local structure around the manganese in the ABCl(3):Mn2+ (A = K, Rb, Ca and B = Mg, Ca, Cd, Sr) chloroperovskite series. EXAFS and XANES experiments carried out in KMgCl3:Mn2+ and RbCaCl3:Mn2+ indicate that the Mn-Cl distances of the MnCl64- complex are 2.51 and 2.53 Angstrom, respectively. These values are very similar to those found in the pure NH4MnCl3 perovskite, R = 2.525 Angstrom, and show that the variations of R along the series do not follow that of the host lattice. The correlation between these measurements and the optical excitation spectra allows us to estimate Mn-Cl bond distances for the whole series with accuracies of about 0.002 Angstrom. The present results are compared with previous structural data reported for the ABF(3):Mn2+ isomorphous fluorides.

Keywords: XAFS (EXAFS AND XANES); CRYSTAL FIELDS; LUMINESCENCE; OPTICAL PROPERTIES; INORGANIC COMPOUNDS

Author(s): VALIENTE R, ARAMBURU JA, BARRIUSO MT, MORENO M

Source: INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY   Volume: 52 Issue: 4  Pages: 1051-1065 

Abstract: MS-Xalpha and SCCEH calculations on the Ag2+ complexes AgF6(4-) and AgC6(4-) (displaying an elongated D4h symmetry) have been carried out for a better understanding of their experimental optical and EPR properties. As salient features, the present work supports that the unpaired electron in AgCl6(4-) spends a little more time on ligands than on Ag2+, in agreement with the previous analysis of EPR and optical data for KCl:Ag2+. Furthermore, the five experimental optical transitions observed in that case are reasonably assigned. The first transition (observed at 12,500 cm-1) is assigned to a jump involving the 5a1g orbital built mainly (approximately 70%) from 3p orbitals of axial ligands, a fact that reflects the distinct level scheme for AgCl6(4-) when compared to that for more ionic complexes. Calculations on AgF6(4-) and AgF4(2-) performed as a function of the equatorial Ag2+ – F- distance led to a reasonable understanding of experimental gyromagnetic and superhyperfine tensors displayed by Ag2+ in fluorides. The different relative decrease undergone by g(parallel-to) – g0 (8%) and g(perpendicular-to) – g0 (28%) on passing from CsCdF3:Ag2+ to RbCdF3:Ag2+ is shown to be consistent with the formation of AgF6(4-) and AgF4(2-) complexes, respectively, related to the different substitutional position of Ag2+ in such lattices. The decrement of about 8.5% experienced by both g(parallel-to) – g0 and g(perpendicular-to) -g0 values on going from CsCdF3:Ag2+ to NaF:Ag2+ is pointed out to reflect the different electrostatic potential (exerted by the rest of the lattice upon the complex) seen by AgF6(4-) embedded in NaCl or perovskite-type lattices. (C) 1994 John Wiley & Sons, Inc.

Keywords: ELECTRON-PARAMAGNETIC-RESONANCE; CHARGE-TRANSFER TRANSITIONS; CUCL42; NI+; ABSORPTION; COPPER(II); IMPURITIES; PARAMETERS; SPECTRUM; LATTICE

Author(s): VALIENTE R, ARAMBURU JA, BARRIUSO MT, MORENO M

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 6 Issue: 24  Pages: 4515-4525

Abstract: The composition of electronic levels as well as optical transitions associated with AgCl6(4-) and AgF6(4-) complexes have been studied through MS-Xalpha and SCCEH calculations performed as a function of equatorial (R(eq)) and axial (R(ax)) metal-ligand distances. The scheme and composition of levels for AgCl6(4-) is rather different from that for AgF6(4-) and other more ionic systems. The first transition for KCl:Ag2+ (observed at 12 500 cm-1) is assigned to a jump involving the 5a1g orbital which is mainly built (about 70%) from 3p orbitals of axial chlorine atoms. Aside from explaining reasonably the five optical bands experimentally observed for KCl:Ag2+, the present work indicates that the first allowed charge-transfer transition of AgF6(4-) would lie in the ultraviolet region and confirms that the unpaired electron in AgCl6(4-) spends a little more time on equatorial ligands than on the central ion. All these results are consistent with a high value (chi = 2.8) for the optical electronegativity of Ag2+. The dependence of electronic transitions (and also of unpaired spin densities f(sigma) and f(s)) on R(eq) and R(ax) is found to be rather similar for both AgF6(4-) and AgCl6(4-) complexes. The relation between such a dependence and the band widths of optical transitions is outlined.

Keywords: CHARGE-TRANSFER TRANSITIONS; X-ALPHA CALCULATIONS; OPTICAL-SPECTRUM; PARAMAGNETIC-RES; SQUARE-PLANAR; COMPLEXES; COPPER; CUCL42; ABSORPTION; PARAMETERS

Author(s): VALIENTE R, DELUCAS MCM, RODRIGUEZ F

Source: JOURNAL OF PHYSICS-CONDENSED MATTER     Volume: 6 Issue: 24  Pages: 4527-4540

Abstract:

This work investigates the polarized charge-transfer (CT) spectra of the Jahn-Teller distorted CuCl6(4-) and CuBr6(4-) complexes formed in Cu2+-doped [N(CH3)4]CdCl3 and [N(CH3)4]CdBr3 CrYstals. The transition energies as well as the dominant polarization of the CT bands along the hexagonal c direction are explained in terms of rhombic D2h distortions. In the bromide complexes, the strong spin-orbit interaction of the Br- ligands leads to additional bands in the low-temperature spectra. Evidence of vibronic couplings to totally symmetric vibrations is found from analysis of the temperature dependence of the band width. The influence of Cu²⁺ impurities in the lattice dynamics is studied through the variation undergone by different spectroscopic parameters in the 10-300 K range. Throughout this work, the high sensitivity of CT transitions for detecting the structural phase transitions exhibited by these crystals is demonstrated. In particular, the enhancement of the first-order character of the ferroelectric phase transition in [N(CH3)4]CdBr3:Cu2+ at T(F) = 157 K, which has been associated with the presence of Cu2+ impurities is worth noting.

Keywords: STRUCTURAL PHASE-TRANSITIONS; TRANSFER SPECTRA; EMISSION DYNAMICS; TRANSFER BANDS; SQUARE-PLANAR; CRYSTALS; ABSORPTION; LATTICE; TMMC; NH4CL-CU2+

Author(s): DELUCAS MCM, RODRIGUEZ F, MORENO M, TRESSAUD A

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 6 Issue: 11  Pages: 6353-6366

Abstract:

Precise photoluminescence measurements have been carried out on RbCdF3:Mn2+ in the 9-300 K range to explore the structural phase transition of the host lattice at T(c) = 124 K using the first (M1) and second (M2) moments of the emission band together with the lifetime r as probes. Aside from showing a sensitivity of M2(T) and r(T) to the phase transition, the present data reveal that the curve M1(T) experiences a small but observable change of slope in the vicinity of T(c). It is demonstrated that this jump, DELTA(t)(partial derivative M1/partial derivative T)p = 0.22 cm-1 K-1, is mainly associated with the increase experienced by the local thermal expansion coefficient below T(c). The importance of both the implicit and explicit contributions to M1 (T) is analysed throughout this work.

Neither the luminescence nor the excitation spectrum shows any evidence of tetragonal distortion in the MnF64- complex. This fact is explained through the R(ax) – R(eq) = 0.2 pm value (where R(ax) and R(eq) are the axial and equatorial Mn2+-F- distances) at 30 K, and the coupling coefficient V(E) = 66 cm-1 pm-1 with the E(g) Jahn-Teller mode of MnF64-. Finally, we have found that this crystal exhibits photochemical reactions under uv light irradiation, leading to the formation of centres that transfer easily to Mn2+ ions. These centres give rise to an intense absorption band at 310 nm, analogously to those previously formed under x-irradiation.

Keywords: TEMPERATURE-DEPENDENCE; RBMNF3; KZNF3; FLUOPEROVSKITES; CRYSTALS; CONSTANT; RBCAF3; CSCAF3

Author(s): DELUCAS MCM, RODRIGUEZ F, MORENO M

Source: PHYSICAL REVIEW B    Volume: 50 Issue: 5  Pages: 2760-2765

Abstract: This work reports the excitation and luminescence spectra at T = 9 K of the Mn2+-doped RbCdF3, RbCaF3, and CsCaF3 fluoroperovskites. A Mn2+ concentration down to 400 ppm has been detected. The present data together with those available for KMgF3:Mn2+, KZnF3:Mn2+, KMnF3, and RbMnF3, allow us to explore the variations of the zero-phonon line (ZPL) and the Stokes shift [associated with the Mn2+ luminescent 4T1g (G) –> 6A1g (S) transition] along the whole series of cubic fluoroperovskites doped with Mn2+ in order to investigate their dependence with the Mn-F distance, R. The experimental variation of the ZPL energy measured in the 206-215 pm range is E(ZPL) = -21 305 + 187R (pm). This strong dependence of ZPL upon R allows one to measure Mn-F distances along the series of fluoroperovskites with accuracies of better than 0.001 angstrom, thus improving by more than one order of magnitude the extended x-ray-absorption fine structure resolution. Furthermore, this sensitivity is extended down to 10(-4) angstrom when we analyze the variations deltaR induced by pressure in a given system. As regards DELTAE(s), the present results on Mn2+-doped fluoroperovskites demonstrate an increase of the Stokes shift when R increases. This behavior is explained to be related to a Gruneisen constant gamma(a1g) corresponding to the symmetric mode of the complex. The value of the constant is greater than 1.9, in agreement with calculations on divalent transition-metal complexes like MnF64- and VF64-. The importance of the present results for the observation of luminescence through the full series of Mn2+-doped fluoroperovskites is also discussed.

Keywords: TEMPERATURE-DEPENDENCE; PHASE-TRANSITIONS; OPTICAL-SPECTRUM; MN-2+-F DISTANCE; PRESSURE; KZNF3; LUMINESCENCE; RBMNF3; PHOTOLUMINESCENCE; K2NAGAF6-CR3+

Author(s): DELUCAS MCM, RODRIGUEZ F, MORENO M

Source: PHYSICA STATUS SOLIDI B-BASIC RESEARCH    Volume: 184 Issue: 1  Pages: 247-265

Abstract: A detailed study of the luminescence and excitation spectra as well as of the lifetime (tau) on Na6MnX8 (X = Cl, Br) precipitates in the 300 to 2 K temperature range is made. In spite of the large distance between nearest Mn2+ ions (0.797 nm for Na6MnCl8), it is demonstrated that even at 2 K luminescence takes place at excitation traps after an exchange induced transfer between close Mn2+ ions. At variance to compounds like MnF2, only one trap (with depth of 350 and 480 cm-1 for X = Cl and Br, respectively) together with killers is mainly responsible for the observed temperature dependence of the luminescence. Aside from the inhomogeneous broadening of the emission band, the extrinsic character of the luminescence is underlined by the experimental Stokes shift observed at 10 K. The analysis of the bandwidth and the vibrational progressions on the 4T1g(G) band is perfectly explained in terms of coupling to the a1g mode. The increase of the Huang-Rhys factor on passing from Na6MnCl8, S(a1g) = 5, to Na6MnBr8, S(a1g = 8, is reasonably explained through a microscopic model. It is demonstrated that the lifetime-temperature dependence is mainly governed by the overlap between the absorption and emission bands. This overlap is estimated subtracting the odd-phonon contribution to the Stokes shift since phonon assistance is not required for excitation transfer through superexchange. The existence of Mn2+-Mn2+ transfer down to 2 K is related to the perfect,cubic symmetry of these compounds, and to a small coupling to the Jahn-Teller e(g) mode. The different thermal shifts experienced by the luminescence and the excitation 4T1g(G) band are also discussed.

Keywords: COUPLED MANGANESE(II) PAIRS; SUZUKI-PHASE; TEMPERATURE-DEPENDENCE; INFRARED LUMINESCENCE; LIFETIME MEASUREMENTS; ELPASOLITE LATTICES; EXCITATION-SPECTRA; OPTICAL-SPECTRA; CRYSTALS; ABSORPTION

Author(s): RODRIGUEZ F, NUNEZ P, DELUCAS MCM

Source: JOURNAL OF SOLID STATE CHEMISTRY     Volume: 110 Issue: 2  Pages: 370-383

Abstract: The polarized optical absorption spectra of the ID manganese (III) fluoride Tl2MnF5 . H2O are investigated in the 9.5 -300 K temperature range. Throughout the work, special emphasis is placed on the correlation between the different spectroscopic parameters and the structural and magnetic properties of the title compound. Three prominent, strongly polarized broadbands are observed at 11,700, 17,500, and 20,800 cm-1, which are assigned to the spin-allowed crystal field bands within the d4 electronic configuration of the Jahn-Teller elongated MnF63- complex (D4h). Both the polarization and the temperature dependence of the oscillator strengths indicate that these bands are electric-dipole-assisted by odd parity vibrations, whereas the spin-forbidden transitions are induced by a pairwise exchange mechanism. A salient feature of the present work is the observation of vibronic progressions to a1g modes of 525, 505, and 375 cm-1 in the low temperature spectra of the 5B1g –> 5A1g and 5B2g bands. The nature of these modes and the experimental Huang-Rhys factors are analyzed in terms of the linear electron-phonon coupling coefficients, which are derived by correlating the optical spectra of several fluorides with the local geometry around the Mn(III). The presence of an exciton magnon peak in the low temperature spectra is also noteworthy. The temperature dependence of the intensity of this peak as well as of the spin-forbidden transitions provides evidence of magnetic ordering below T(N) = 28 K and allows us to estimate an intrachain exchange constant of J = 11 cm-1. (C) 1994 Academic Press, Inc

Keywords: D-D SPECTRUM; MAGNETIC-STRUCTURES; PHASE-TRANSITIONS; COMPOUND; MN-2+; FLUOROMANGANATES(III); MONOHYDRATE; IONS; CS

Author(s): DELUCAS MCM, RODRIGUEZ F, GUDEL HU, FURER N

Source: JOURNAL OF LUMINESCENCE      Volume: 60 Issue: 1  Pages: 581-584 

Abstract: This work reports the excitation and luminescence spectra of several ABCl3:Mn2+ crystals. The variations in the optical spectra along this series are explained by changes in the Mn-Cl distance, R, of the MnCl64- complex as a consequence of the chemical pressure. The lattice relaxation around Mn2+ and the R dependence of the Stokes shift are analysed in detail.

Keywords: COUPLED MANGANESE(II) PAIRS; SPECTRA; DEPENDENCE; PHOTOLUMINESCENCE; TRANSITION; NH4MNCL3

Author(s): VALIENTE R, DELUCAS MCM, ESPESO JI, RODRIGUEZ F

Source: SOLID STATE COMMUNICATIONS      Volume: 86 Issue: 10  Pages: 663-666 

Abstract: The optical properties of [(CH3)4N]2MBr4 (M = Cd and Mn) crystals are investigated. Both crystals are positive biaxial with the principal axes of the optic indicatrix n(alpha),n(beta) and n(gamma) along the a, b and c orthorhombic directions, respectively. The birefringence, optic angle and refractive indexes were measured at room temperature. The monoclinic domain structure observed below Pmcn –> P12(1)/c1 phase transition temperature is also analyzed.

Keywords: PHASE-TRANSITIONS

Author(s): DELUCAS MCM, RODRIGUEZ F

Source: JOURNAL OF PHYSICS-CONDENSED MATTER      Volume: 5 Issue: 16  Pages: 2625-2642 

Abstract: This work investigates the influence of Cu2+ impurities on the luminescence properties Of TMA2MnBr4:Cu2+ as well as the local structure and orientation of the CuBr42- complexes formed, by means of the excitation and luminescence spectra, lifetime measurements and polarized optical absorption spectroscopy in the 10-300 K temperature range. It is demonstrated that the presence of an intense Br –> Cu2+ charge transfer (CT) band at 555 nm strongly favours a direct energy transfer from Mn2+ to the non-luminescent Cu2+ impurities The influence of this energy transfer on the Mn2+ luminescence intensity, lifetime and bandshape is analysed as a function of the Cu2+ concentration. The results are compared with previous ones obtained in one-dimensional Cu2+-doped TMAMnCl3 and TMAMnBr3 crystals. Two x, y-polarized bands at 18 000 and 28 400 cm-1, and two z-polarized bands at 23 800 and 36 100 cm-1 are observed in the CT spectra Of TmA2MnBr4:Cu2+. Their transition energies as well as their polarization are explained in terms of D2d symmetry distortions of the CuBr42- tetrahedra. We also analyse the triplet structure observed in the first CT band which is associated with the tetrahedral 2T1 CT state, which is split by the effect of both the static D2d distortion and the large spin-orbit coupling of the Br- ligands. The absence of discontinuities in the evolution of the CuBr42- CT bands with temperature supports the finding that no structural phase transition occurs below 270 K in these crystals.

Keywords: YELLOW THERMOCHROMIC MODIFICATIONS; TETRAHEDRAL CUCL42 ANIONS; ELECTRON-PARAMAGNETIC-RES; ONE-DIMENSIONAL LATTICE; DISCRETE SQUARE-PLANAR; PHASE-TRANSITION; BIS(N-METHYLPHENETHYLAMMONIUM) TETRACHLOROCUPRATE(II); TETRAMETHYLAMMONIUM TETRABROMOMANGANATE; EMISSION DYNAMICS; ROOM-TEMPERATURE

Author(s): DELUCAS MCM, RODRIGUEZ F, MORENO M

Source: JOURNAL OF PHYSICS-CONDENSED MATTER  Volume: 5 Issue: 9  Pages: 1437-1446  

Abstract: Precise photoluminescence measurements on an RbCaF3:Mn2+ sample containing Only 400 ppm of Mn2+ have been carried out in the 10-300 K temperature range. The results are compared with those obtained in other fluoroperovskites doped with Mn2+. The analysis of the 6A1g(S) –> 4T1g(G) excitation peak at room temperature leads to a Mn2+-F- distance R = 213.3 pm which is close to that derived from the experimental isotropic superhyperfine constant, A(s). The plot of the first moment of the emission band, MI, against temperature reveals a slight but sensible change of slope at T = 193 K which is associated with the O(h)1 –> D4h18 structural phase transition of the host lattice. Furthermore, at T = 40 K, M1 undergoes an abrupt increase of approximately 100 cm-1. This fact supports the existence of another phase transition involving an increase DELTAR/R congruent-to 0.2% upon cooling, and thus a Situation which is Similar to that detected in the structural phase transition of KMnF3 at T(c3) = 81.5 K. To our knowledge this is the first time that clear evidence of both phase transitions in RbCaF3 has been achieved through an optical probe. Finally the variation of the 4A1g(G), 4E(g)(G) peak, E3, along the fluoroperovskite series is analysed.

Keywords: PEROVSKITE FLUORIDES; MN-2+-F DISTANCE; OPTICAL-SPECTRUM; RBCDF3; MN2+; DEPENDENCE; LATTICES; CRYSTALS; RBMNF3; KMGF3

Author(s): BRENOSA AG, RODRIGUEZ F, MORENO M

Source: SOLID STATE COMMUNICATIONS  Volume: 85 Issue: 2  Pages: 135-140

Abstract:

he transition temperatures and the small hysteresis associated to the beta –> delta phase transition in NH4Cl1-xBrx crystals (x less-than-or-equal-to 0.05) have been measured through the temperature dependence of the e(u)(pi + sigma, Cl) –> a1g* (approximately 3z2 – r2) peak corresponding to the CuCl4(NH3)22- impurity centre used as probe.

The hysteresis DELTAT = 0.2 +/- 0.1 K obtained for NH4Cl compares well with the values derived from dilatometric and specific heat measurements but not with those coming from quasi elastic light scattering or ultrasonic data. The present results support the equivalence of P-T and X-T phase diagrams around of x = 0.

Keywords: SINGLE-CRYSTALS; TRANSFER BANDS; HIGH-PRESSURE; NH4CL; ND4CL; COMPLEXES; EXPANSION

Author(s): DELUCAS MCM, RODRIGUEZ F

Source: FERROELECTRICS    Volume: 125    Issue: 1-4    Pages: 159-164    

Abstract: The local geometry and the orientation of the CuCl42- complexes formed in TMA2MnCl4:Cu2+ are studied through the polarized Charge Transfer (CT) spectra in the 10 – 300 K temperature range. The analysis of these spectra indicates that CuCl42-complexes display distortions of D2d symmetry along only one of the tetrahedron S4 axes, with bond angles of 124-degrees. The structural phase transitions undergone by this crystal mainly involve reorientational motions of the CuCl42- complexes. Below the P112(1)/n–>P12(1)/c1 phase transition temperature, 171 K, the CT spectra experience the highest variations due to the formation of domains.

KeyWords Plus: SINGLE-CRYSTALS; TETRAMETHYLAMMONIUM

Author(s): DELUCAS MCM, RODRIGUEZ F, MORENO M

Source: PHYSICA STATUS SOLIDI B-BASIC RESEARCH    Volume: 172    Issue: 2    Pages: 719-731    

Abstract: The aggregation states of Mn2+ in KBr:Mn2+ doped crystals are investigated using optical techniques. Excitation and luminescence spectra as well as lifetimes are obtained as a function of temperature of different crystals. It is demonstrated that at least three different Mn2+ precipitates are formed inside the KBr. Two of them contain MnBr64- units with slightly distorted octahedral symmetries, while the third one involves two non-equivalent tetrahedral MnBr42- units. These centers exhibit a broad green luminescence band.

KeyWords Plus: COUPLED MANGANESE(II) PAIRS; ENERGY-TRANSFER; SUZUKI-PHASE; MAGNETIC-PROPERTIES; ALKALI-HALIDES; TRANSITION; SPECTRA; NACL; ABSORPTION; EMISSION

Author(s): RODRIGUEZ F, SAL JCG, MORENO M

Source: PHYSICA B    Volume: 180    Pages: 531-533    Part: Part A    

Abstract: This work examines the way of extracting reliable information on the shape and size of microcrystalline precipitates with cubic morphologies from SANS experiments out of the Guinier region. The information can be derived from the I(q) dependences for different crystallographic directions as I is proportional to q(-n) and n is a function of both the q direction and the precipitate shape.

Author(s): DELUCAS MCM, RODRIGUEZ F, ARAMBURU JA

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 3    Issue: 45    Pages: 8945-8957    

Abstract: The charge transfer (c-r) spectra of the CUCl42- complex formed in TMA2MnCl4:Cu2+ single crystals are investigated in the 10-300 K temperature range. Two chi, gamma-polarized bands at 24 300 and 33 700 cm-1, and two z-polarized ones at 29 500 and 41 000 cm-1 are assigned to CT transitions of the CuCl42-. These transition energies, as well as the degree of polarization of the CT bands, are explained in terms of Jahn-Teller distortions of D2d symmetry. The ms-Xa calculations performed on this complex support this view. By means of these data, both the trans-Cl-Cu-Cl distortion angle and the orientation of CuCl₄^2- in the lattice are determined. The sensitivity of CT bands of CuCl4(2-) for detecting the structural phase transition sequence of this crystal is analysed as well. A noteworthy finding of this work concerns the profound changes experienced by CT bands below the monoclinic P112l/n –> P12l/c1 phase transition temperature at 171 K. The spectra become nearly isotropic as a consequence of the formation of structural domains. These results are compared with those obtained in pure chlorocuprates such as Cs2CuCl4 and TMA₂CuCl₄.

KeyWords Plus: ELECTRON-PARAMAGNETIC-RES; PHASE-TRANSITION; HYDROSTATIC-PRESSURE; SINGLE-CRYSTALS; TRANSFER BANDS; TETRACHLOROCUPRATE(II); COMPLEXES; (N(CH3)4)2MNCL4; PARAMETERS; FEATURES

Author(s): BRENOSA AG, MORENO M, RODRIGUEZ F, COUZI M

Source: PHYSICAL REVIEW B    Volume: 44    Issue: 18    Pages: 9859-9863    

Abstract: The temperature dependence of the (a) e(u) (pi + sigma, Cl) –> a1g* (approximately 3z2-r2) charge-transfer transition, nu-CT; (b) e(g)* –> a1g* crystal-field transition, VCF; (C) Cu-NH3 and Cu-Cl A1g stretching mode frequencies (termed as nu-1 and nu-2, respectively) corresponding to the Cu-Cl4(NH3)2(2-) center in NH4Cl have been measured in the 100-300-K range. Below T(c), nu-CF and especially nu-CT undergo a red shift ascribed to the increase of the Cu2+-Cl- equatorial distance, R(eq), of the center although the lattice parameter a decreases. This anomalous relaxation is strongly supported by Raman data, giving variations DELTA-nu-1 = 7 +/- 1 cm-1 and DELTA-nu-2 = 11 +/- 3 cm-1 in the 243-213-K range. Based on these data, the variations undergone by R(eq) and the axial Cu+-NH3 distance R(ax) in such a temperature range are estimated to be DELTA-R(eq) congruent-to + 2.0 +/- 0.6 pm, DELTA-R(ax) congruent-to -0.40 +/- 0.05 pm. The present results emphasize the usefulness of standard spectroscopic tools for detecting this unusual local relaxation which lies beyond the resolution limit of the extended x-ray-absorption fine-structure technique. To our knowledge this is the first time that evidence of such a phenomenon is reported.

KeyWords Plus: CHARGE-TRANSFER TRANSITIONS; TEMPERATURE-DEPENDENCE; THERMAL-EXPANSION; OPTICAL-SPECTRUM; FINE-STRUCTURE; CUCL42; ABSORPTION; COMPLEXES; DISTANCE; CRYSTALS

Author(s): RODRIGUEZ F, RIESEN H, GUDEL HU

Source: JOURNAL OF LUMINESCENCE    Volume: 50    Issue: 2    Pages: 101-110   

Abstract: The luminescence spectra of KZnF3: Mn2+ and KMgF3: Mn2+ are presented and analysed. At T = 5 K, both spectra consist of one zero phonon line from which a phonon sideband centered at about 600 nm is formed. The existence of the dynamical Jahn-Teller (JT) effect in the first 4T1g excited state is evidenced through a reduction of the spin-orbit splitting displayed by the zero phonon lines. JT energies of 530 and 450 cm-1 associated with electron-phonon couplings carrying e(g) type distortions around Mn2+ are estimated for KZnF3: Mn2+ and KMgF3: Mn2+, respectively. Vibronic sidebands are interpreted in terms of lattice modes rather than local vibrations of the MnF6(4-) complex. The magnetic and electric dipole character of the 4T1g –> 6A1g transition is analysed by the temperature dependence of the luminescence spectra and their lifetimes.

KeyWords Plus: TRANSITION-METAL IONS; HIGH-RESOLUTION MCD; TEMPERATURE-DEPENDENCE; EXCITATION-SPECTRA; OPTICAL-SPECTRUM; FLUORIDE; STATE; RBMNF3; MGF2; MN2+

Reprint Address: RODRIGUEZ, F (reprint author), UNIV CANTABRIA, SECC CIENCIA MAT, DCITT &

Author(s): RODRIGUEZ F, SAL JCG, MORENO M, DEGEYER A, JANOT C

Source: PHYSICAL REVIEW B    Volume: 43    Issue: 10    Pages: 7519-7526    Part: Part A    

Abstract: Suzuki-phase precipitates formed in NaCl:Mn2+ have been studied by small-angle neutron scattering. The experiments, carried out on single crystals of NaCl with different Mn2+ concentrations, indicate the usefulness of this technique for detecting precipitates and for determining their shape and size. A salient feature of the present work is the anisotropic character of the neutron-scattering intensity, which provides a way for determining the size of the percipitate out of the Guinier region. The analysis of the intensity decay allows us to detect precipitate shapes displaying slight modifications with respect to the perfect cube.

KeyWords Plus: SUZUKI-PHASE; DOPED NACL; CRYSTALS; SPECTROSCOPY

Author(s): BRENOSA AG, RODRIGUEZ F, MORENO M

Source: FERROELECTRICS    Volume: 106    Pages: 187-192

Author(s): DELUCAS CM, RODRIGUEZ F, DANCE JM, MORENO M, TRESSAUD A

Source: JOURNAL OF LUMINESCENCE    Volume: 48-9    Pages: 553-557    Part: Part 2   

Abstract: Photoluminescence, lifetime, optical absorption and EPR measurements on Rb2KGaF6:Cr3+ have been carried out in the 10-300 K temperature range. The emission spectrum at 10 K shows a very rich vibrational structure which can be mainly explained through the vibrational modes of the CrF6(3-) complex. Comparison of the experimental vibrational frequencies and zero-phonon line with those for K2NaAlF6:Cr3+ indicates that the Cr3+-F- distance is about 1.5 pm larger for Rb2KGaF6:Cr3+. The present data clearly demonstrate the existence of a first order phase transition at 129 K (upon cooling) with an involved hysteresis DELTA-T = 6 K although the local geometry around Cr3+ in the noncubic phase remains essentially octahedral.

KeyWords Plus: STRUCTURAL PHASE-TRANSITIONS; D-D SPECTRUM; RB2KMIIIF6 ELPASOLITES; OPTICAL-SPECTRUM; K2NAGAF6-CR3+; SPECTROSCOPY; DEPENDENCE; MN-2+; CR3+

Author(s): DELUCAS MCM, RODRIGUEZ F, MORENO M

Source: FERROELECTRICS    Volume: 109    Pages: 21-26

Author(s): DELUCAS CM, RODRIGUEZ F, MORENO M

Abstract. As-grown NaBr : Mn» crystals have been investigated using photostimulated luminescence, EPR and optical absorption techniques in the 6 K-300 K temperature range. Through them the formation of a precipitated phase involving Mn2+, whose magnetic phase transition temperature is well below 6 K, is established. This conclusion then supports the presence of the Suzuki phase previously inferred from Raman data. The value Q* (Alg) = 130 t 7 cm-‘ derived at 14 K from the vibrational progression in the 4T,(G) band  concurs with thecorresponding Ramanvalue. It isshown that, only through the 4Al(G),’E (G) peak, lying at 419.6 nm at 14 K for the present case, the presence of MnBri- cannot be definitely established. This situation is however improved by looking at the Stokes  shifts, the 10 Dqvalues and also the optical bandwidths. The analysis of 10 Dqand O(A,,) suggests that the Mnzt-Br- distance R is higher than that for MnBr,or Cdox sMn, IsBrzH. owever an increase AR = 4.8 pm with respect to MnBr, does not fully account for the ‘anomalous’ ‘A,(G), 4E(G) peak position in the present case where a value J = 0.014 cm-‘ has been derived for the exchange constant from the EPR bandwidth AHpp = 117 G. Attention has been paid to the influence of temperature changes on the optical peak positions  as well as to the effects of a quenching at 600 «C. Although AHpp becomes 2.5 times higher, no evidence is obtained on the formation of isolated Mn2- ions after quenching.

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 2    Issue: 7    Pages: 1891-1903   

Author(s): DELUCAS CM, RODRIGUEZ F

Source: REVIEW OF SCIENTIFIC INSTRUMENTS    Volume: 61    Issue: 1    Pages: 23-26    Part: Part 1 

Author(s): DELUCAS MCM, RODRIGUEZ F

Source: JOURNAL OF PHYSICS-CONDENSED MATTER    Volume: 1    Issue: 26    Pages: 4251-4256

Author(s): RODRIGUEZ F, MORENO M, SAL JCG, JANOT C

Source: PHYSICA B    Volume: 156    Pages: 33-35

Author(s): RODRIGUEZ F, MORENO M, DANCE JM, TRESSAUD A

Source: SOLID STATE COMMUNICATIONS    Volume: 69    Issue: 1    Pages: 67-71    

Abstract: The influence of temperature on the position of crystal-field peaks of RbMnF₃ and KMnF₃ has been investigated in the 14–550 K range. Upon warming in the 200–550 K range, the ⁴T_1g(G) peak dependent on 10 Dq experiences a total blue shift of 470 cm^-1. It is shown that only 40% of this shift arises from thermal expansion effects, the rest being due to the explicit (∂E/∂T)_v term. The positive sign of this term is associated to higher vibration frequencies for the ⁴T_1g(G) state (belonging mainly to the t⁴e configuration) than for ⁶A_1g(S) as a result of smaller Mn²⁺-F^– distances for that excited state. Below 200 K the present data reflect the existence of magnetic and structural (only for KMnF₃) phase transitions in the compounds.

Author(s): MORENO M, RODRIGUEZ F, SAL JCG

Source: JOURNAL DE PHYSIQUE    Volume: 49    Issue: C-8    Pages: 1811-1812    Part: Part 3   

Author(s): BRENOSA AG, RODRIGUEZ F, MORENO M

Abstract. Charge transfer(cr)spectradue tosmallamounts(-lOO ppm)~f[CuBr,(NH,)~]’ centres in the NH,Br lattice have been investigated in the 300-14 K temperature range. When upon cooling the y+ 6 phase transition occurs the  assigned e,(n + U; Br)-+ a;, and e,(u + R; Br)- aC peaks experience abrupt red shifts of 700 and 1200 cm-’ respectively. stressing the great sensitivity of CT peaks for detecting that transition. Those red shifts support the assertion that when the y -+ b transition is produced the Cu*’-Br- distance increases though the average lattice parameter of NH,Br decreases abruptly by 2.4 pm. This is the first time that the first-order sluggish y+ 6 transition in NH,Br and the hysteresis effects involved have been clearly detected using an impurity as probe. Moreover CT bands also detect the phase transition at 234 K and the possibility for the crystal to remain in a metastable tetragonal phase below 78 K.

Source: JOURNAL OF PHYSICS C-SOLID STATE PHYSICS    Volume: 21    Issue: 17    Pages: L623-L630    

Author(s): SAL JCG, RODRIGUEZ F, MORENO M, THOLENCE JL

Source: PHYSICAL REVIEW B    Volume: 37    Issue: 1    Pages: 454-458    

Author(s): GOMEZSAL JC, MORENO M, RODRIGUEZ F, REVEX A, THOLENCE JL

Abstract. Specific heat measurements for an as-grown NaCl: Mn2+ crystal (Mn2′ concentration: 17000 ppm) containing the Suzuki phase have been carried out in the 0.1-10 K temperature range. The contribution due to the Suzuki phase shows a very sharp peak at 0.2 K which strongly supports the existence of an antiferromagnetic phase transition. The experimental entropy change associated with such a peak, equal to 16.6 J K-‘ mol-‘, is very  close to the molar magnetic entropy expected when S = B ions are involved. To our knowledge this is the first time that relevant specific heat data on precipitated phases in insulator materials have been reported.

Source: JOURNAL OF PHYSICS C-SOLID STATE PHYSICS    Volume: 20    Issue: 19    Pages: L421-L424

Author(s): RODRIGUEZ F, MORENO M, ROUSSEAU JJ

Source: CRYSTAL LATTICE DEFECTS AND AMORPHOUS MATERIALS    Volume: 16    Issue: 1-4    Pages: 161-167 

Author(s): RODRIGUEZ F, MORENO M, TRESSAUD A, CHAMINADE JP

Source: CRYSTAL LATTICE DEFECTS AND AMORPHOUS MATERIALS    Volume: 16    Issue: 1-4    Pages: 221-225 

Author(s): RODRIGUEZ F, BRENOSA AG, ARAMBURU JA, MORENO M, CALLEJA JM

Abstract. We have explored the centre I1 formed in NH,CI: Cu2+ crystals by means of optical absorption and Raman spectroscopy. The crystal-field spectrum of centre I1 is shown to be composed of three bands, consistent with its Ddhsymmetry. An analysis of peak positions suggests NHrCu2+ and CI–Cu2+ distances close to 1.9 and 2.4 Arespectively. At 14 K the crystal-field bands assigned as b&y) + aI8 and e,(xz; yz) + a,, show nice vibronic progressions corresponding to P = 400 * 15 cm-I. Raman spectroscopy reveals the presence of a sharp peak corresponding to S2, = 445 cm-‘. Both P and Q, frequencies are associated with the NHTCu2+ symmetric stretching of the (CUCI,(NH,),]~- unit. This difference reflects higher NHrCu2+ distances for the excited states due to the promotion of an additional electron to the strongly anti-bonding alg level.

Source: JOURNAL OF PHYSICS C-SOLID STATE PHYSICS    Volume: 20    Issue: 26    Pages: L641-L647  

Author(s): BRENOSA AG, MORENO M, RODRIGUEZ F

Source: SOLID STATE COMMUNICATIONS    Volume: 63    Issue: 6    Pages: 543-547 

Author(s): GOMEZSAL JC, THOLENCE JL, RODRIGUEZ F, RAVEX A, MORENO M, ARAMBURU JA

Source: CRYSTAL LATTICE DEFECTS AND AMORPHOUS MATERIALS    Volume: 16    Issue: 1-4    Pages: 235-239   

Author(s): RODRIGUEZ F, MORENO M

Abstract. The thermal expansion of the Mn*’-F- bond in KZnF3 : Mn2+ in the 15-300 K temperature range has been studied from the variations undergone by the crystal-field spectrum. It is shown that the thermal expansion effects are clearly more significant for the Mn2+-F- distance than for the Zn2′-F- distance of the perfect lattice. In this way the linear thermal expansion coefficient (Y for Mn*+-F- is found to be (Y = (38 k 14)10-6 K-‘ at room temperature to be compared with (Y = (15 * 0.7)10-6 K-I for the host lattice. To our knowledge this is the first time that data on thermal expansion effects around an impurity have been reported

Source: JOURNAL OF PHYSICS C-SOLID STATE PHYSICS    Volume: 19    Issue: 23    Pages: L513-L517    

Author(s): RODRIGUEZ F, MORENO M

Source: SOLID STATE COMMUNICATIONS    Volume: 58    Issue: 10    Pages: 701-704   

Abstract: It is stressed in this work that for ionic crystals doped with luminescent cations the progressive red shift of the emission peak energy when temperature increases is a direct proof of the formation of microcrystalline precipitated phases inside the host lattice. As an example the case of as grown RbCl:Mn²⁺ crystals is reported and analyzed in detail.

Author(s): RODRIGUEZ F, MORENO M

Source: JOURNAL OF CHEMICAL PHYSICS    Volume: 84    Issue: 2    Pages: 692-697    

Abstract: Wehave measured by means of photoluminescence the energy of crystal-fieldpeaks for RbCdF₃: Mn²⁺ and KZnF₃: Mn²⁺ where the valueof the Mn²⁺–F⁻ distance, R, derived by EXAFS is R=2.13±0.01Å and R=2.08±0.01 Å, respectively. From these data and thosefor RbMnF₃ and KMnF₃ we have studied the dependence onR of the B, C, and 10 Dq parameters forthe MnF₆^4- complex. This analysis reveals that within the experimentalerrors, B and C are constant in the range 2.07<R<2.14Å, in agreement with recent self-consistent calculations for MnF₆^4-, whichalso predict that 10 Dq=KR⁻ⁿ, where K and n areconstant. The present study confirms this dependence, n being 4.4which is also in accord to the theoretical predictions. Thebest values of R derived from optical spectra are foundto be R=2.141±0.004 Å (for RbCdF₃: Mn²⁺) and R=2.075±0.004 Å(for KZnF₃: Mn²⁺). The present analysis also points out thatby measuring the changes induced on the optical spectrum ofMnF₆^{4-  in} a given lattice we can detect changes inthe Mn²⁺–F⁻ distance down to 10⁻³ Å. In this waywe have derived the difference, R, between R at roomtemperature and at 77 K for KZnF₃: Mn²⁺. The obtainedvalue R=(9±1)10⁻³ Å is in agreement with the one R=(10±3.5)10⁻³ Å derived previously from the variations undergone by the isotropicsuperhyperfine constant A_s. Finally the present results are compared tothose for some complexes of Eu²⁺, Co²⁺, Ni²⁺, and Cr³⁺. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.