Polymorphism of praseodymium orthovanadate under high pressure

Zircon-type $\mathrm{PrV}{\mathrm{O}}₄$ has been studied at high pressures and room temperature by means of synchrotron powder x-ray diffraction. At room temperature, we observed the previously known zircon-to-monazite phase transition at 5.5(4) GPa and a second phase transition from monazite to a monoclinic structure at 12.7(8) GPa, which we identified as a $\mathrm{PbW}{\mathrm{O}}₄$-III-type phase. This conclusion is supported by our ab initio calculations, which also predict a scheelite-type phase to be stable at high pressure. Motivated by this finding, we subjected zircon-type $\mathrm{PrV}{\mathrm{O}}₄$ samples to high pressure (7 GPa) and temperature (600, 800, and 1000 \ifmmode^\circ\else\textdegree\fi{}C) in a Paris-Edinburgh large volume press cell. As a result, we could synthesize a scheelite-type phase of $\mathrm{PrV}{\mathrm{O}}₄$, which was recovered in a metastable state at room conditions, and the unit-cell parameters and the atomic positions have been determined. We also report a substantial increase of the scheelite proportion with time by comparing synthesis of different durations, at the same pressure-temperature conditions. The transition mechanisms of the zircon-, monazite-, and scheelite-type structures as well as the role of kinetic barriers will be discussed. Information on the compressibility of different polymorphs will be reported too.