MOLECULAR-DYNAMICS SIMULATION OF PRESSURE-INDUCED CRYSTALLINE-TO-AMORPHOUS TRANSITION IN SOME CORNER-LINKED POLYHEDRAL COMPOUNDS

Molecular-dynamics calculations are carried out on quartz, AlPO4, and LiKSO4, 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 T1OT2 angle (T1=Si,Al,Li;T2=Si,P,S) is near 124-degrees, 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.