Phase transition systematics in ${\mathrm{BiVO}}_4$ by means of high-pressure–high-temperature Raman experiments

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 $T_{F-\mathrm{Sch}}\left(\text{K}\right)=-166\left(8\right)P\left(\text{GPa}\right)+528\left(5\right)$. The zircon-to-scheelite, irreversible, first-order phase transition takes place at $T_{Z-\mathrm{Sch}}\left(\text{K}\right)=-107\left(8\right)P\left(\text{GPa}\right)+690\left(10\right)$. 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.

Figure 1

Symmetry assignment of Raman modes in the fergusonite phase by comparison of single crystal polarized spectra with the spectrum of the sample in powder form. In single crystal spectra $B_g$ modes are not allowed. The spectra have been shifted for clarity. Continuous (dashed) lines at the bottom correspond to the ab initio calculation for the $A_g$ ($B_g$) modes. The peak labeled with a star is a glitch originated by the edge filter.

Figure 2

Raman spectra corresponding to the fergusonite polymorph. Upper panel: Selected high-pressure spectra. Numbers next to the spectra indicate pressure in GPa, either in the upstroke or downstroke. Spectra correspond to the different experiments, either with a 16:3:1 methanol:ethanol:water as pressure transmitting medium or Ne (underlined pressures) Lower panel: Selected spectra at different temperatures (K).

Figure 3

Raman spectra corresponding to the zircon polymorph. Upper panel: Selected high-pressure spectra. Numbers next to the spectra indicate pressure in GPa, (d) either in the upstroke or downstroke. Methanol-ethanol-water is used as pressure transmitting medium. Lower panel: Selected spectra at different temperatures (K).

Figure 4

Upper panel: Pressure dependence of the Raman modes of fergusonite ${\text{BiVO}}_4$. The three different symbols represent three separate experiments. Squares correspond to an experiment in which the sample is in powder form and a 16:3:1 methanol-ethanol-water mixture is used as pressure transmitting medium. Diamonds experiment was carried out with a single crystal and Ne as pressure transmitting medium. Triangles represent an experiment where fergusonite powder and Ne were used. Filled (hollow) symbols were obtained in the upstroke (downstroke). Lower panel: Temperature dependence of the Raman modes of fergusonite structure.

Figure 5

Raman modes of the zircon polymorph. Upper panel: Pressure dependence. Lower panel: Temperature dependence.

Figure 6

Selected patterns of lattice vibrations at the gamma point in zircon (Z), scheelite (S) and fergusonite (F). Large, medium and small spheres represent Bi, V, and O atoms, respectively. We represent only the first neighbor cell of V and Bi atoms. Modes labeled $\nu$, $T$, and $R$ are derived from the internal, translational, and rotational modes of the ${\text{VO}}_4$ tetrahedra.

Figure 7

Analysis of the order parameter as pressure and/or temperature is varied. $\mathrm{\Delta }$ is proportional to the order parameter [Eqs. (8) and (9)]. Pressure and temperature are combined as in Eq. (6). Symbols correspond to different experiments. Squares: ambient pressure, temperature variation; circles and stars: ambient temperature, pressure variation; triangles: $T=331\text{K}$, pressure variation; rhombs: $T=342$ K, pressure variation; hexagons: $T=391$ K, pressure variation.