Molecular-dynamics simulation of pressure-induced crystalline-to-amorphous transition in some corner-linked polyhedral compounds

Molecular-dynamics calculations are carried out on quartz, ${\mathrm{AlPO}}_4$, and ${\mathrm{LiKSO}}_4$, which are composed of corner-linked tetrahedra and are known to display crystalline-to-amorphous transitions at 15, 12, and 13 GPa, respectively, at 300 K. The transition occurs when the ${\mathit{T}}_1$O${\mathit{T}}_2$ angle (${\mathit{T}}_1$=Si,Al,Li; ${\mathit{T}}_2$=Si,P,S) is near 124°, and the nearest oxygen atoms approach within their van der Waals separation distance. Consequently some of the cations acquire a higher coordination. The existing experimental data are in agreement with these results. A quasiharmonic lattice-dynamical calculation as a function of pressure suggests a zone-boundary phonon instability prior to the phase transition.