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We report here high pressure-high temperature Raman experiments performed on BiV O4. We have characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nano powder). The experimental results are supported by ab-initio calculations, which in addition provide the vibrational patterns. The temperature and pressure behavior of the fergusonite lattice modes reflects the distortions associated with the ferroelastic instability. The linear coefficients of the zircon phase are in sharp contrast with the behavior observed in the fergusonite phase. The boundary of the fergusonite to scheelite second order phase transition is given by T F −Sch (K) = −166(8)P (GP a) + 528(5). The zircon to scheelite, irreversible, first order phase transition takes place at T Z−Sch (K) = −107(8)P (GP a) + 690(10). We found evidence of additional structural changes around 15.7 GP a, which in the downstroke were found to be not reversible. We have analyzed the anharmonic contribution to the wavenumber shift in fergusonite using an order parameter. The introduction of a critical temperature depending both on temperature and pressure allows description of the results of all the experiments in a unified way.

We have studied the occurrence of phase transitions in two polymorphs of BiVO 4 under high-pressure and high-temperature conditions by means of X-ray diffraction measurements. The fergusonite polymorph undergoes a phase transition at 1.5(1) GPa and room temperature into a tetragonal scheelite-type structure. The same transition takes place at 523(1) K and ambient pressure. A second phase transition takes place at room temperature under compression at 16(1) GPa. The transition is from the tetragonal scheelite structure to a monoclinic structure (space group P2 1 /c). All observed phase transitions are reversible. The zircon polymorph counterpart also transforms under compression into the scheelite-type structure. In this case, the transitions take place at 4.3(1) GPa and room temperature and at 653(1) K and ambient pressure. The zircon−scheelite transition is nonreversible. The experiments support that the fergusonite−scheelite transformation is a second-order transition and that the zircon−scheelite transformation is a first-order transition. Finally, we have also determined the compressibility and the thermal expansion of the fergusonite, scheelite, and zircon phases.