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ABSTRACT: In this paper, we report ab initio calculations of the vibrational and structural properties of ZnAl2O4 and ZnGa2O4 spinel structures. The calculated vibrational modes at zero pressure, at the Γ point and the complete phonon spectrum of both compounds are presented. Also, we report our findings for the high-pressure structure
High Pressure Research 02/2013; 29:573-577. · 0.78 Impact Factor
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ABSTRACT: The electronic structure of four ternary-metal oxides containing isolated
vanadate ions is studied. Zircon-type YVO4, YbVO4, LuVO4, and NdVO4 are
investigated by high-pressure optical-absorption measurements up to 20 GPa.
First-principles calculations based on density-functional theory were also
performed to analyze the electronic band structure as a function of pressure.
The electronic structure near the Fermi level originates largely from molecular
orbitals of the vanadate ion, but cation substitution influence these
electronic states. The studied ortovanadates, with the exception of NdVO4,
undergo a zircon-scheelite structural phase transition that causes a collapse
of the band-gap energy. The pressure coefficient dEg/dP show positive values
for the zircon phase and negative values for the scheelite phase. NdVO4
undergoes a zircon-monazite-scheelite structural sequence with two associated
band-gap collapses.
Journal of Applied Physics 02/2013; 110:043723. · 2.17 Impact Factor
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ABSTRACT: We have performed experimental and theoretical studies of the structural
stability of YAsO4 and YCrO4 at high pressures. X-ray diffraction experiments
together with ab initio total-energy and lattice-dynamics calculations have
allowed us to completely characterize a pressure-induced structural phase
transition from the zircon to the scheelite structure in both compounds.
Furthermore, total-energy calculations have been performed to check the
relative stabilities of different candidate structures at different pressures
and allow us to propose for YAsO4 the zircon \rightarrow scheelite \rightarrow
SrUO4-type sequence of structures. In this sequence, sixfold arsenic
coordination is attained for the SrUO4-type structure above 32 GPa. The whole
sequence of transitions is discussed in comparison with YVO4, YPO4, YNbO4,
YMoO4, and YTaO4. Also a comparative discussion of lattice-dynamics properties
is presented. The band-gap for YAsO4 and YCrO4 and band structure for YAsO4 are
also reported. Finally, the room-temperature equation of state of different
compounds is also obtained.
01/2012;
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O. Gomis,
R. Vilaplana,
F. J. Manjon,
E. Perez-Gonzalez,
J. Lopez-Solano,
P. Rodriguez-Hernandez, A. Munoz,
D. Errandonea,
J. Ruiz-Fuertes,
A. Segura,
D. Santamaria-Perez,
I. M. Tiginyanu,
V. V. Ursaki
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ABSTRACT: High-pressure optical absorption and Raman scattering measurements have been performed in defect chalcopyrite (DC) CdGa2Se4 up to 22 GPa during two pressure cycles to investigate the pressure-induced order-disorder phase transitions taking place in this ordered-vacancy compound. Our measurements reveal that on decreasing pressure from 22 GPa, the sample does not revert to the initial phase but likely to a disordered zinc blende (DZ) structure the direct bandgap and Raman-active modes of which have been measured during a second upstroke. Our measurements have been complemented with electronic structure and lattice dynamical ab initio calculations. Lattice dynamical calculations have helped us to discuss and assign the symmetries of the Raman modes of the DC phase. Additionally, our electronic band structure calculations have helped us in discussing the order-disorder effects taking place above 6-8 GPa during the first upstroke. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3675162]
Journal of Applied Physics 01/2012; 111. · 2.17 Impact Factor
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ABSTRACT: The optical-absorption edge of single crystals of CaWO4, SrWO4, BaWO4, and
PbWO4 has been measured under high pressure up to ~20 GPa at room temperature.
From the measurements we have obtained the evolution of the band-gap energy
with pressure. We found a low-pressure range (up to 7-10 GPa) where
alkaline-earth tungstates present a very small Eg pressure dependence (-2.1 <
dEg/dP < 8.9 meV/GPa). In contrast, in the same pressure range, PbWO4 has a
pressure coefficient of -62 meV/GPa. The high-pressure range is characterized
in the four compounds by an abrupt decrease of Eg followed by changes in
dEg/dP. The band-gap collapse is larger than 1.2 eV in BaWO4. We also
calculated the electronic-band structures and their pressure evolution.
Calculations allow us to interpret experiments considering the different
electronic configuration of divalent metals. Changes in the pressure evolution
of Eg are correlated with the occurrence of pressure-induced phase transitions.
The band structures for the low- and high-pressure phases are also reported. No
metallization of any of the compounds is detected in experiments nor is
predicted by calculations.
07/2011;
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ABSTRACT: X-ray diffraction and Raman-scattering measurements on cerium vanadate have
been performed up to 12 and 16 GPa, respectively. Experiments reveal that at
5.3 GPa the onset of a pressure-induced irreversible phase transition from the
zircon to the monazite structure. Beyond this pressure, diffraction peaks and
Raman-active modes of the monazite phase are measured. The zircon to monazite
transition in CeVO4 is distinctive among the other rare-earth orthovanadates.
We also observed softening of external translational Eg and internal B2g
bending modes. We attributed it to mechanical instabilities of zircon phase
against the pressure-induced distortion. We additionally report
lattice-dynamical and total-energy calculations which are in agreement with the
experimental results. Finally, the effect of non-hydrostatic stresses on the
structural sequence is studied and the equations of state of different phases
are reported.
Physical Review B 05/2011; · 3.69 Impact Factor
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ABSTRACT: The availability of ab initio electronic calculations and the concomitant
techniques for deriving the corresponding lattice dynamics have been profusely
used for calculating thermodynamic and vibrational properties of
semiconductors, as well as their dependence on isotopic masses. The latter have
been compared with experimental data for elemental and binary semiconductors
with different isotopic compositions. Here we present theoretical and
experimental data for several vibronic and thermodynamic properties of CuGa2, a
canonical ternary semiconductor of the chalcopyrite family. Among these
properties are the lattice parameters, the phonon dispersion relations and
densities of states (projected on the Cu, Ga, and S constituents), the specific
heat and the volume thermal expansion coefficient. The calculations were
performed with the ABINIT and VASP codes within the LDA approximation for
exchange and correlation and the results are compared with data obtained on
samples with the natural isotope composition for Cu, Ga and S, as well as for
isotope enriched samples.
02/2011;
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Physical Review B. 01/2011; 83(19):195208.
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Rosario Vilaplana,
D. Santamaria-Perez,
O. Gomis,
F. J. Manjon,
J. Gonzalez,
A. Segura, A. Munoz,
P. Rodriguez-Hernandez,
E. Perez-Gonzalez,
V. Marin-Borras,
V. Munoz-Sanjose,
C. Drasar,
V. Kucek
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ABSTRACT: The structural and vibrational properties of bismuth selenide (Bi2Se3) have been studied by means of x-ray diffraction and Raman scattering measurements up to 20 and 30 GPa, respectively. The measurements have been complemented with ab initio total-energy and lattice dynamics calculations. Our experimental results evidence a phase transition from the low-pressure rhombohedral (R-3m) phase (alpha-Bi2Se3) with sixfold coordination for Bi to a monoclinic C2/m structure (beta-Bi2Se3) with sevenfold coordination for Bi above 10 GPa. The equation of state and the pressure dependence of the lattice parameters and volume of alpha and beta phases of Bi2Se3 are reported. Furthermore, the presence of a pressure-induced electronic topological phase transition in alpha-Bi2Se3 is discussed. Raman measurements evidence that Bi2Se3 undergoes two additional phase transitions around 20 and 28 GPa, likely toward a monoclinic C2/c and a disordered body-centered cubic structure with 8-fold and 9- or 10-fold coordination, respectively. These two high-pressure structures are the same as those recently found at high pressures in Bi2Te3 and Sb2Te3. On pressure release, Bi2Se3 reverts to the original rhombohedral phase after considerable hysteresis. Symmetries, frequencies, and pressure coefficients of the Raman and infrared modes in the different phases are reported and discussed.
Physical Review B 01/2011; 84. · 3.69 Impact Factor
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ABSTRACT: We have studied by means of high-pressure x-ray diffraction the structural stability of Ni2Mo3N, Co3Mo3N, and Fe3Mo3N. We also report ab initio computing modeling of the high-pressure properties of these compounds, Pd2Mo3N, and Pt2Mo3N. We have found that the nitrides remain stable in the ambient-pressure cubic structure at least up to 50 GPa and determined their equation of state. All of them have a bulk modulus larger than 300 GPa. Single-crystal elastic constants have been calculated in order to quantify the stiffness of the investigated nitrides. We found that they should have a Vickers hardness similar to that of cubic spinel nitrides like gamma-Si3N4 Comment: 25 pages, 6 figures, 3 tables
11/2010;
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ABSTRACT: We report ab initio calculations of the electronic band structure, the corresponding optical spectra, and the phonon dispersion relations of trigonal alpha-HgS (cinnabar). The calculated dielectric functions are compared with unpublished optical measurements by Zallen and coworkers. The phonon dispersion relations are used to calculate the temperature and isotopic mass dependence of the specific heat which has been compared with experimental data obtained on samples with the natural isotope abundances of the elements Hg and S (natural minerals and vapor phase grown samples) and on samples prepared from isotope enriched elements by vapor phase transport. Comparison of the calculated vibrational frequencies with Raman and ir data is also presented. Contrary to the case of cubic beta-HgS (metacinnabar), the spin-orbit splitting of the top valence bands at the Gamma-point of the Brillouin zone (Delta_0) is positive, because of a smaller admixture of 5d core electrons of Hg. Calculations of the lattice parameters, and the pressure dependence of Delta_0 and the corresponding direct gap E_0~2eV are also presented. The lowest absorption edge is confirmed to be indirect. Comment: 13 pages, 15 figures
07/2010;
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J. Ruiz-Fuertes,
D. Errandonea,
R. Lacomba-Perales,
A. Segura,
J. Gonzalez,
F. Rodriguez,
F. J. Manjon,
S. Ray,
P. Rodriguez-Hernandez, A. Munoz,
Zh. Zhu,
C. Y. Tu
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ABSTRACT: We study the effects of pressure on the structural, vibrational, and magnetic behavior of cuproscheelite. We performed powder x-ray diffraction and Raman spectroscopy experiments up to 27 GPa as well as ab initio total-energy and lattice-dynamics calculations. Experiments provide evidence that a structural phase transition takes place at 10 GPa from the low-pressure triclinic phase (P-1) to a monoclinic wolframite-type structure (P2/c). Calculations confirmed this finding and indicate that the phase transformation involves a change in the magnetic order. In addition, the equation of state for the triclinic phase is determined: V0 = 132.8(2) A3, B0 = 139 (6) GPa and = 4. Furthermore, experiments under different stress conditions show that non-hydrostatic stresses induce a second phase transition at 17 GPa and reduce the compressibility of CuWO4, B0 = 171(6) GPa. The pressure dependence of all Raman modes of the triclinic and high-pressure phases is also reported and discussed. Comment: 33 pages, 9 figures, 5 tables
05/2010;
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ABSTRACT: This paper reports an investigation on the phase diagram and compressibility of wolframite-type tungstates by means of x-ray powder diffraction and absorption in a diamond-anvil cell and ab initio calculations. X-ray diffraction experiments show that monoclinic wolframite-type MgWO <sub>4</sub> suffers at least two phase transitions, the first one being to a triclinic polymorph with a structure similar to that of CuWO <sub>4</sub> and FeMoO <sub>4</sub> -II . The onset of each transition is detected at 17.1 and 31 GPa. In ZnWO <sub>4</sub> the onset of the monoclinic-triclinic transition has been also found at 16.7 GPa. This transition does not involve any change in the atomic coordination as confirmed by x-ray absorption measurements. These findings are supported by density-functional theory calculations, which predict the occurrence of additional transitions upon further compression. Calculations have been also performed for wolframite-type MnWO <sub>4</sub> , which is found to have an antiferromagnetic configuration. In addition, our study reveals details of the local-atomic compression in MgWO <sub>4</sub> and ZnWO <sub>4</sub> . In particular, below the transition pressure the ZnO <sub>6</sub> and equivalent polyhedra tend to become more regular, whereas, the WO <sub>6</sub> octahedra remain almost unchanged. Fitting the pressure-volume data we obtained the equation of state for the low-pressure phase of MgWO <sub>4</sub> and ZnWO <sub>4</sub> . These and previous results on MnWO <sub>4</sub> and CdWO <sub>4</sub> are compared wit-
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h the calculations. The compressibility of wolframite-type tungstates is also systematically discussed. Finally Raman spectroscopy measurements and lattice dynamics calculations are presented for MgWO <sub>4</sub> .
Journal of Applied Physics 05/2010; · 2.17 Impact Factor
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ABSTRACT: We have performed a theoretical and experimental study of the structural stability of terbium phosphate at high pressures. Theoretical ab initio total-energy and lattice-dynamics calculations together with x-ray diffraction experiments have allowed us to completely characterize a phase transition at similar to 9.8 GPa from the zircon to the monazite structure. Furthermore, total-energy calculations have been performed to check the relative stability of 17 candidate structures at different pressures and allow us to propose the zircon -> monazite -> scheelite -> SrUO4-type sequence of stable structures with increasing pressure. In this sequence, sixfold P coordination is attained for the SrUO4-type structure above 64 GPa. The whole sequence of transitions is discussed in association with the high-pressure structural behavior of oxides isomorphic to TbPO4.
Physical Review B. 01/2010; 81(14).
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ABSTRACT: We have measured the specific heat of zincblende ZnS for several isotopic compositions and over a broad temperature range (3 to 1100 K). We have compared these results with calculations based on ab initio electronic band structures, performed using both LDA and GGA exchange- correlation functionals. We have compared the lattice dynamics obtained in this manner with experimental data and have calculated the one-phonon and two-phonon densities of states. We have also calculated mode Grueneisen parameters at a number of high symmetry points of the Brillouin zone. The electronic part of our calculations has been used to investigate the effect of the 3d core electrons of zinc on the spin-orbit splitting of the top valence bands. The effect of these core electrons on the band structure of the rock salt modification of ZnS is also discussed. Comment: 33pages, 16 Figures, submitted to Phys. Rev. B
11/2009;
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ABSTRACT: We report ab initio calculations of the electronic band structure and the phonon dispersion relations of the zincblende-type mercury chalcogenides (beta-HgS, HgSe, and HgTe). The latter have been used to evaluate the temperature dependence of the specific heat which has been compared with experimental data. The electronic band structure of these materials has been confirmed to have an inverted direct gap of the alpha-tin type, which makes HgSe and HgTe semimetallic. For beta-HgS, however, our calculations predict a negative spin-orbit splitting which restores semiconducting properties to the material in spite of the inverted gap. We have calculated the spin-orbit induced linear terms in k which appear at the Gamma_8 valence bands. We have also investigated the pressure dependence of the crystal structure and the phonons. Comment: 32 pages, 15 figures
08/2009;
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ABSTRACT: Tetragonal scheelite-type CaWO <sub>4</sub> nanocrystals recently prepared by a hydrothermal method show an enhancement of its structural symmetry with the decrease in nanocrystal size. The analysis of the volume dependence of the structural parameters in CaWO <sub>4</sub> nanocrystals with the help of ab initio total-energy calculations shows that the enhancement of the symmetry in the scheelite-type nanocrystals is a consequence of the negative pressure exerted on the nanocrystals; i.e., the nanocrystals are under tension. Besides, the behavior of the structural parameters in CaWO <sub>4</sub> nanocrystals for sizes below 10 nm suggests an onset of a scheelite-to-zircon phase transformation in good agreement with the predictions from our ab initio calculations. CaWO <sub>4</sub> nanocrystals exhibit a reconstructive-type mechanism for the scheelite-to-zircon phase transition that seems to follow the tetragonal path that links both structures. This result is in contrast with the mechanism recently proposed for this transition in bulk ZrSiO <sub>4</sub> where the transition goes through an intermediate monoclinic phase.
Journal of Applied Physics 06/2009; · 2.17 Impact Factor
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ABSTRACT: Ab initio calculations, based on the density functional theory, of the structural properties of MnWO4 wolframite compound are reported. We obtain the equilibrium volume from an equation of state with the anti-ferromagnetic (AF) and ferromagnetic configurations, AF being the lowest energy state, and the structural properties of the wolframite structure at zero pressure are obtained. We also study the wolframite structure up to a pressure of 31 GPa, and the pressure evolution of structural parameters is found to be in good agreement with the available experimental data.
High Pressure Research 02/2009; 29:578-581. · 0.78 Impact Factor
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ABSTRACT: Room-temperature Raman scattering was measured in ZnWO4 up to 45 GPa. We report the pressure dependence of all the Raman-active phonons of the low-pressure wolframite phase. As pressure increases new Raman peaks appear at 30.6 GPa due to the onset of a reversible structural phase transition to a distorted monoclinic b-fergusonite-type phase. The low- and high-pressure phases coexist from 30.6 GPa to 36.5 GPa. In addition to the Raman measurements we also report ab initio total-energy and lattice-dynamics calculations for the two phases. These calculations helped us to determine the crystalline structure of the high-pressure phase and to assign the observed Raman modes in both the wolframite and b-fergusonite phases. Based upon the ab initio calculations we propose the occurrence of a second phase transition at 57.6 GPa from the b-fergusonite phase to an orthorhombic Cmca phase. The pressure evolution of the lattice parameters and the atomic positions of wolframite ZnWO4 are also theoretically calculated and an equation of state reported.
10/2008;
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ABSTRACT: We performed high-pressure angle dispersive x-ray diffraction measurements on Fe5Si3 and Ni2Si up to 75 GPa. Both materials were synthesized in bulk quantities via a solid-state reaction. In the pressure range covered by the experiments, no evidence of the occurrence of phase transitions was observed. On top of that, Fe5Si3 was found to compress isotropically, whereas an anisotropic compression was observed in Ni2Si. The linear incompressibility of Ni2Si along the c-axis is similar in magnitude to the linear incompressibility of diamond. This fact is related to the higher valence-electron charge density of Ni2Si along the c-axis. The observed anisotropic compression of Ni2Si is also related to the layered structure of Ni2Si where hexagonal layers of Ni2+ cations alternate with graphite-like layers formed by (NiSi)2- entities. The experimental results are supported by ab initio total-energy calculations carried out using density functional theory and the pseudopotential method. For Fe5Si3, the calculations also predicted a phase transition at 283 GPa from the hexagonal P63/mcm phase to the cubic structure adopted by Fe and Si in the garnet Fe5Si3O12. The room-temperature equations of state for Fe5Si3 and Ni2Si are also reported and a possible correlation between the bulk modulus of iron silicides and the coordination number of their minority element is discussed. Finally, we report novel descriptions of these structures, in particular of the predicted high-pressure phase of Fe5Si3 (the cation subarray in the garnet Fe5Si3O12), which can be derived from spinel Fe2SiO4 (Fe6Si3O12). Comment: 44 pages, 13 figures, 3 Tables
Physical Review B 03/2008; · 3.69 Impact Factor
