Phase diagram and thermodynamic properties of AIPOinf4/inf based on first-principles calculations and the quasiharmonic approximation

We calculated the phase diagram of AlPOinf4/inf up to 15 GPa and 2,000 K and investigated the thermodynamic properties of the high-pressure phases. The investigated phases include the berlinite, moganite-like, AlVOinf4/inf, P2inf1/inf/c , and CrVOinf4/inf phases. The computational methods used include density functional theory, density functional perturbation theory, and the quasiharmonic approximation. The investigated thermodynamic properties include the thermal equation of state, isothermal bulk modulus, thermal expansivity, and heat capacity. With increasing pressure, the ambient phase berlinite transforms to the moganite-like phase, and then to the AlVOinf4/inf and P2inf1/inf/c phases, and further to the CrVOinf4/inf phase. The stability fields of the AlVOinf4/inf and P2inf1/inf/c phases are similar in pressure but different in temperature, as the AlVOinf4/inf phase is stable at low temperatures, whereas the P2inf1/inf/c phase is stable at high temperatures. All of the phase relationships agree well with those obtained by quench experiments, and they support the stabilities of the moganite-like, AlVOinf4/inf, and P2inf1/inf/c phases, which were not observed in room-temperature compression experiments.