Phase diagram and thermodynamic properties of AIPO4 based on first-principles calculations and the quasiharmonic approximation

We calculated the phase diagram of (hbox {AlPO}_{4}) 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, (hbox {AlVO}_{4},, P2_1/c), and (hbox {CrVO}_{4}) 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 (hbox {AlVO}_{4}) and (P2_1/c) phases, and further to the (hbox {CrVO}_{4}) phase. The stability fields of the (hbox {AlVO}_{4}) and (P2_1/c) phases are similar in pressure but different in temperature, as the (hbox {AlVO}_{4}) phase is stable at low temperatures, whereas the (P2_1/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, (hbox {AlVO}_{4}), and (P2_1/c) phases, which were not observed in room-temperature compression experiments.