Elastic instabilities have been used to explain the occurrence of amorphization in both α-quartz (${\mathrm{SiO}}_2$) and α-berlinite (${\mathrm{AlPO}}_4$). However, there is a dynamical instability at (1/3,1/3,0) in the Brillouin zone preceding the elastic instability for both structures which implies that distortion of a 3×3×1 supercell will form a more stable crystal structure. Simulations of distorted supercells of size 3×3×1 resulted in the collapse of the structures below the elastic instability pressures for both α-quartz and α-berlinite. A subtle difference between them was that the eigenvectors for phosphorus were smaller than those of silicon or aluminum and on ‘‘amorphization’’ the ${\mathrm{PO}}_4$ units in α-berlinite remained completely intact with only the aluminum changing coordination. This resulted in amorphization around the ${\mathrm{PO}}_4$ units with no P-O-P links formed and a return to a crystalline phase on the release of pressure for α-berlinite, which was not found in the simulations of α-quartz.

• Received 9 June 1995

DOI:https://doi.org/10.1103/PhysRevB.52.13306