Mechanical instabilities in AlPO4

Pressure-induced amorphization has been observed in isostructural oxides such as α-quartz (SiO2), GeO2, and α-berlinite (AlPO4). Here we focus on the mechanical behavior of α-berlinite under pressure. We have computed the second-order elastic constants of α-berlinite and applied the Born stability criteria. Our analysis reveals an elastic instability associated with soft shear modes. We have also computed the phonon spectrum of α-berlinite as a function of pressure. A soft phonon mode corresponding to the shear instability is observed at the Brillouin-zone edge. We have analyzed the displacements of cations associated with the soft phonon modes in α-quartz and α-berlinite. They exhibit the same qualitative behavior. This strongly suggests that the mechanism of amorphization is similar in these materials.