Competing dynamical and lattice instabilities in RVO4 rare-earth vanadium oxides under high pressure

Abstract

Zircon-type ${R\mathrm{VO}}_{4}$ compounds (where R = rare-earth atom) constitute a family of ternary oxides which are abundant in both nature and the industrial field. Their structural systematics upon compression has been widely studied. According to the observed systematics, the high-pressure crystal structure is determined by the rare-earth cation size. Nonetheless, the mechanisms involved in the phase transitions remain relatively unexplored. Thus, we gathered the experimental data reported in the literature and compared it to our ab initio calculations, including not only enthalpy, but also the frequency of lattice modes and the value of elastic constants. Our study reveals systematic trends, where high-pressure phases appear only if they are thermodynamically stable and are always preceded by a mechanical or dynamical instability, which counteracts the effects of kinetic barriers in the zircon-to-monazite or -scheelite phase transition, respectively.