6533b827fe1ef96bd128641f

RESEARCH PRODUCT

Phase transition systematics in BiVO4 by means of high-pressure–high-temperature Raman experiments

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We report here high-pressure--high-temperature Raman experiments performed on ${\text{BiVO}}_{4}$. We characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nanopowder). 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 to the behavior observed in the fergusonite phase. The boundary of the fergusonite-to-scheelite second-order phase transition is given by ${T}_{F\ensuremath{-}\mathrm{Sch}}(\text{K})=\ensuremath{-}166(8)P(\text{GPa})+528(5)$. The zircon-to-scheelite, irreversible, first-order phase transition takes place at ${T}_{Z\ensuremath{-}\mathrm{Sch}}(\text{K})=\ensuremath{-}107(8)P(\text{GPa})+690(10)$. We found evidence of additional structural changes around 15.7 GPa, which in the downstroke were found to be not reversible. We analyzed the anharmonic contribution to the wave-number shift in fergusonite using an order parameter. The introduction of a critical temperature depending both on temperature and pressure allows for a description of the results of all the experiments in a unified way.