Electronic, Vibrational, and Structural Properties of the Natural Mineral Ferberite (FeWO4): A High-Pressure Study.

This paper reports an experimental high-pressure study of natural mineral ferberite (FeWO4) up to 20 GPa using diamond-anvil cells. First-principles calculations have been used to support and complement the results of the experimental techniques. X-ray diffraction patterns show that FeWO4 crystallizes in the wolframite structure at ambient pressure and is stable over a wide pressure range, as is the case for other wolframite AWO4 (A = Mg, Mn, Co, Ni, Zn, or Cd) compounds. No structural phase transitions were observed for FeWO4, in the pressure range investigated. The bulk modulus (B0 = 136(3) GPa) obtained from the equation of state is very close to the recently reported value for CoWO4 (131(3) GPa). According to our optical absorption measurements, FeWO4 has an indirect band gap that decreases from 2.00(5) eV at ambient pressure to 1.56(5) eV at 16 GPa. First-principles simulations yield three infrared-active phonons, which soften with pressure, in contrast to the Raman-active phonons. These results agree with Raman spectroscopy experiments on FeWO4 and are similar to those previously reported for MgWO4. Our results on FeWO4 are also compared to previous results on other wolframite-type compounds.

High-pressure polymorph of Co3P2O8: phase transition to an olivine-related structure.

The monoclinic polymorph of Co3P2O8 (space group P21/c), isomorphic to farringtonite (Mg3P2O8) type orthophosphates, was studied up to 21 GPa using synchrotron powder X-ray diffraction and density-functional theory simulations to investigate the influence of pressure in the crystal structure. This study revealed a pressure induced structural phase transition for monoclinic cobalt phosphate, Co3P2O8, and the details of crystal structure of the new high-pressure polymorph were delineated. The evolution of XRD pattern with pressure indicate that the onset of a phase transition occurs around 2.90(5) GPa, and the low- and high- pressure phases coexist up to 10.3(1) GPa. The high-pressure phase also has a monoclinic lattice (space group P21/c), and a discontinuous change of unit-cell volume of about 6.5% occurs at the transition. A reorganization of atomic positions with a change in the cobalt coordination sphere occurs in the phase transition. Notably, the high-pressure polymorph has a defect-olivine-type structure like chopinite-type orthophosphates. Using a third-order Birch-Murnaghan equation of state, the bulk moduli of the low pressure (LP) phase (75(2) GPa) and high-pressure (HP) phase (92(2) GPa) were determined. For the low-pressure polymorph, the principal axes of compression and their compressibility were also determined. Density-functional theory calculations also provided the frequencies of Raman- and infrared-active modes which can be used for mode assignment in future experiments.

PbV2O6 under compression: near zero-linear compressibility and pressure-induced change in vanadium coordination.

This study presents evidence that lead metavanadate, PbV2O6, is a material with zero-linear compressibility, which maintains its crystal size in one crystallographic direction even under external pressures of up to 20 GPa. The orthorhombic polymorph of PbV2O6 (space group Pnma) was studied up to 20 GPa using synchrotron powder X-ray diffraction, Raman spectroscopy, and density-functional theory simulations to investigate its structural and vibrational evolution under compression. Up to this pressure we find no evidence of any structural phase transitions by any diagnostic technique, however, a progressive transformation of the coordination polyhedron of vanadium atoms is revealed which results in the zero-linear compressibility. High-pressure Raman experiments enabled the identification and symmetry assignation of all 54 zone-centre Raman-active modes as well as the calculation of their respective pressure coefficients. Three independent high-pressure powder X-ray diffraction experiments were performed using different pressure-transmitting media (Ne, 4 : 1 methanol-ethanol mixture, and silicone oil). The results show a high anisotropic behaviour in the linear compressibility of the crystallographic axes. The PbV2O6 bulk modulus of 86.1(9) GPa was determined using a third-order Birch-Murnaghan equation of state. The experimental results are supported by ab initio density-functional theory calculations, which provide vibrational patterns, unit-cell parameters, and atomic positions. These calculations also reveal that, unlike MgV2O6 and ZnV2O6, the band gap of PbV2O6 closes with pressure at a rate of -54 meV GPa-1 due to the contribution of the Pb 6s orbital to the top of the valence band.

High-Pressure Behavior of Ca2SnO4, Sr2SnO4, and Zn2SnO4.

The pressure-induced structural evolution of Ca2SnO4, Sr2SnO4, and Zn2SnO4 has been characterized by powder X-ray diffraction up to 20 GPa using the ALBA synchrotron radiation source and density functional theory calculations. No phase transition was observed in Ca2SnO4 and Zn2SnO4 in the investigated pressure range. The observation in Zn2SnO4 solves contradictions existing in the literature. In contrast, a phase transition was observed in Sr2SnO4 at a pressure of 9.09 GPa. The transition was characterized as from the ambient-condition tetragonal polymorph (space group I4/mmm) to the low-temperature tetragonal polymorph (space group P42/ncm). The linear compressibility of crystallographic axes and room-temperature pressure-volume equation of state are reported for the three compounds studied. Calculated elastic constants and moduli are also reported as well as a systematic discussion of the high-pressure behavior and bulk modulus of M2SnO4 stannates.

Equations of State and Crystal Structures of KCaPO4, KSrPO4, and K2Ce(PO4)2 under High Pressure: Discovery of a New Polymorph of KCaPO4.

We have studied by means of angle-dispersive powder synchrotron X-ray diffraction the structural behavior of KCaPO4, SrKPO4, and K2Ce(PO4)2 under high pressure up to 26, 25, and 22 GPa, respectively. For KCaPO4, we have also accurately determined the crystal structure under ambient conditions, which differs from the structure previously reported. Arguments supporting our structural determination will be discussed. We have found that KCaPO4 undergoes a reversible phase transition. The onset of the transition is at 5.6 GPa. It involves a symmetry decrease. The low-pressure phase is described by space group P3̅m1 and the high-pressure phase by space group Pnma. For KSrPO4 and K2Ce(PO4)2, no evidence of phase transitions has been found up to the highest pressure covered by the experiments. For the three compounds, the linear compressibility for the different crystallographic axes and the pressure-volume equation of states are reported and compared with those of other phosphates. The three studied compounds are among the most compressible phosphates. The results of the study improve the knowledge about the high-pressure behavior of complex phosphates.

High-Pressure X-ray Diffraction Study of Orthorhombic Ca2Zr5Ti2O16.

The orthorhombic polymorph of Ca2Zr5Ti2O16 (space group Pbca) has been studied by powder X-ray diffraction under high pressures up to 30 GPa using synchrotron radiation. We have found evidence of a structural phase transition at 12-13 GPa. The phase transition causes an enhancement of the crystal symmetry. The high-pressure phase is tetragonal, being described by space group I41/acd. The space groups of the high- and low-pressure phases have a group/subgroup relationship. However, the phase transition is accompanied by a discontinuous change in the unit-cell volume, indicating that the phase transition can be classified as first order. We have also performed density functional theory calculations. These simulations support the occurrence of the orthorhombic-to-tetragonal transition. The pressure-volume equation of state and axial compressibilities have been determined for both polymorphs. The results are compared with previous studies in related oxides.

Pressure-induced phase transitions and electronic properties of Cd2V2O7.

We report a density-functional theory study of the structural and electronic properties of Cd2V2O7 under high-pressure conditions. The calculations have been performed by using first-principles calculations with the CRYSTAL program. The occurrence of two structural phase transitions, at 0.3 and 10.9 GPa, is proposed. The crystal structure of the different high-pressure phases is reported. Interestingly a cubic pyrochlore-type structure is predicted to stabilize under compression. The two phase transitions involve substantial changes in the coordination polyhedra of Cd and V. We have also determined the compressibility and room-temperature equation of state of the three polymorphs of Cd2V2O7. According to our systematic electronic band-structure calculations, under ambient conditions Cd2V2O7 is an indirect wide band-gap material with a band-gap energy of 4.39 eV. In addition, the pressure dependence of the band gap has been determined. In particular, we have found that after the second phase transition the band gap decreases abruptly to a value of 2.56 eV.

Density-functional study of pressure-induced phase transitions and electronic properties of Zn2V2O7.

We report a study of the high-pressure behavior of the structural and electronic properties of Zn2V2O7 by means of first-principle calculations using the CRYSTAL code. Three different approaches have been used, finding that the Becke-Lee-Yang-Parr functional is the one that best describes Zn2V2O7. The reported calculations contribute to the understanding of previous published experiments. They support the existence of three phase transitions for pressures smaller than 6 GPa. The crystal structure of the different high-pressure phases is reported. We have also made a systematic study of the electronic band-structure, determining the band-gap and its pressure dependence for the different polymorphs. The reported results are compared to previous experimental studies. All the polymorphs of Zn2V2O7 have been found to have a wide band gap, with band-gap energies in the near-ultraviolet region of the electromagnetic spectrum.