First-principles prediction of ferroelastic phase transition in AlPO4

First-principles calculations on AlPO4 predicted the stability of a stishovite-like phase and the existence of a ferroelastic phase transition between this phase and m-CaCl2 phase. The ferroelastic phase transition is consistently supported from the point view of energy, spontaneous strain, tilt angles of octahedra, and behaviour of soft-modes. The soft modes are B 1 g in stishovite-like phase and Ag in m-CaCl2 phase. The static transition occurs at pressure 45.3 GPa, which was judged from the character of soft modes, and phase stability fields were investigated up to high temperatures. For computational methods, this study calculated total energies with density functional theory (DFT), phonon properties with density functional perturbation theory (DFPT), and high-pressure high-temperature phase boundaries within quasiharmonic approximation (QHA).