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 influences these electronic states. The studied orthovanadates, 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 shows 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.

The electronic band-structure and band-gap dependence on the d character of A2+ cation in AWO4 wolframite-type oxides is investigated for different compounds (A = Mg, Zn, Cd, and Mn) by means of optical-absorption spectroscopy and first-principles density-functional calculations. High pressure is used to tune their properties up to 10 GPa by changing the bonding distances establishing electronic to structural correlations. The effect of unfilled d levels is found to produce changes in the nature of the band gap as well as its pressure dependence without structural changes. Thus, whereas Mg, Zn, and Cd, with empty or filled d electron shells, give rise to direct and wide band gaps, Mn, with a half-filled d shell, is found to have an indirect band gap that is more than 1.6 eV smaller than for the other wolframites. In addition, the band gaps of MgWO4, ZnWO4, and CdWO4 blue-shift linearly with pressure, with a pressure coefficient of approximately 13~eV/GPa. However, the band gap of multiferroic MnWO4 red-shifts at $-${}22~meV/GPa. Finally, in MnWO4, absorption bands are observed at lower energy than the band gap and followed with pressure based on the Tanabe-Sugano diagram. This study allows us to estimate the crystal-field variation with pressure for the MnO6 complexes and how it affects their band-gap closure.

We report systematic ab initio calculations of the electronic band structure, phonon dispersion relation, and the structural characterization of FeF2 in the rutile (P42/mnm) structure as well as in several high-pressure phases by means of the generalized gradient approximation (GGA)+U approximation. Using the phonon dispersion relations, we calculated the Gibbs free energy and evaluated the phase transitions at 300 K, at which most experimental measurements are performed. Calculated Raman and infrared vibrational modes, lattice parameters, and electronic structure for all considered crystalline structures are compared with available experimental data. Our calculations show that at 5.33 GPa, the FeF2 undergoes a second-order proper ferroelastic phase transition, rutile $\rightarrow${} CaCl2-type structure. This result is supported by the softening of the elastic shear module Cs in the rutile phase, the softening (hardening) of the B1g (Ag) Raman active mode in the rutile (CaCl2-type) structure near the transition pressure, and the decrease of the square of the spontaneous strain ess from the CaCl2-type structure. This demonstrates that the rutile $\rightarrow${} CaCl2-type phase transition is driven by the coupling between the Raman active B1g mode and shear modulus Cs. At 8.22 GPa, the CaCl2-type structure undergoes a first-order phase transition to the Pbca phase, a distorted fcc Pa3\ifmmode\bar\else\textasciimacron\fi{} phase with a volume reduction of $\Delta${}V$\approx${}7%, as reported in experiments. Upon further increase of the pressure, the Pbca phase transforms to a Fmmm phase othorhombic center-type structure at $\sim${}20.38 GPa, with $\Delta${}V$\approx${}2.5%. Finally, at 25.05 GPa, there is a phase transition to the orthorhombic cotunnite structure (Pnma space group), with $\Delta${}V$\approx${}5.8%, which is stable up to 45 GPa, the largest considered pressure. The coordination number for the Fe ion in each phase is 6, 6, 6, 8, and 9 for rutile, CaCl2-type, Pbca, Fmmm, and cotunnite structures, respectively. The evolution of the band gap, phonon frequencies, and magnetic moment of Fe ion as a function of the applied pressure is reported for all studied phases. The exchange constants J1, J2, and J3, calculated for rutile and the lowest Gibbs free-energy high-pressure phases, are reported.

Raman scattering measurements and lattice-dynamics calculations were performed on magnesium tungstate (MgWO${}_4$) 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 MgWO${}_4$ and isomorphic compounds is examined. Under quasihydrostatic conditions, a phase transition was found at 26 GPa in MgWO${}_4$. The high-pressure phase is tentatively assigned to a triclinic structure similar to that of CuWO${}_4$. 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 MgWO${}_4$, MnWO${}_4$, FeWO${}_4$, CoWO${}_4$, NiWO${}_4$, ZnWO${}_4$, and CdWO${}_4$.

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 MgWO4 suffers at least two phase transitions, the first one being to a triclinic polymorph with a structure similar to that of CuWO4 and FeMoO4-II. The onset of each transition is detected at 17.1 and 31 GPa. In ZnWO4 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 MnWO4, which is found to have an antiferromagnetic configuration. In addition, our study reveals details of the local-atomic compression in MgWO4 and ZnWO4. In particular, below the transition pressure the ZnO6 and equivalent polyhedra tend to become more regular, whereas, the WO6 octahedra remain almost unchanged. Fitting the pressure-volume data we obtained the equation of state for the low-pressure phase of MgWO4 and ZnWO4. These and previous results on MnWO4 and CdWO4 are compared with the calculations. The compressibility of wolframite-type tungstates is also systematically discussed. Finally Raman spectroscopy measurements and lattice dynamics calculations are presented for MgWO4.

The phase transitions driven by pressure together with the pressure dependence of structural, electronic, vibrational, and magnetic properties of MnF ₂ have been investigated using DFT.