Stability of FeVO₄ under Pressure: An X-ray Diffraction and First-Principles Study

Abstract

The high-pressure behavior of the crystalline structure FeVO₄ has been studied by means of X-ray diffraction using a diamond-anvil cell and first-principles calculations. The experiments were carried out up to a pressure of 12.3 GPa, until now the highest pressure reached to study an FeVO₄ compound. High-pressure X-ray diffraction measurements show that the triclinic P1̅ (FeVO₄-I) phase remains stable up to ≈3 GPa; then a first-order phase transition to a new monoclinic polymorph of FeVO₄ (FeVO₄-II') with space group C2/ m is observed, having an α-MnMoO₄-type structure. A second first-order phase transition is observed around 5 GPa toward the monoclinic ( P2/ c) wolframite-type FeVO₄-IV structure, which is stable up to 12.3 GPa in coexistence with FeVO₄-II'. The unit cell volume reductions for the first and second phase transitions are Δ V = -8.5% and -13.1%. It was observed that phase transitions are irreversible and both high-pressure phases remain stable once the pressure is released. Calculations were performed at the level of the generalized gradient approximation plus Hubbard correction (GGA+ U) and with the hybrid Heyd-Scuseria-Ernzerhof (HSE06) exchange-correlation functional in order to have a good representation of the pressure behavior of FeVO₄. We found that theoretical results follow the pressure evolution of structural parameters of FeVO₄, in good agreement with the experimental results. Also, we analyze FeVO₄-II (orthorhombic Cmcm, CrVO₄-type structure) and -III (orthorhombic Pbcn, α-PbO₂-type structure) phases and compare our results with the literature. Going beyond the experimental results, we study some possible post-wolframite phases reported for other compounds and we found a phase transition for FeVO₄-IV to raspite (monoclinic P2₁/ c) type structure (FeVO₄-V) at 36 GPa (Δ V = -8.1%) and a further phase transition to the AgMnO₄-type (monoclinic P2₁/ c) structure (FeVO₄-VI) at 66.5 GPa (Δ V = -3.7%), similar to the phase transition sequence reported for InVO₄.