Dynamical instabilities in α-quartz and α-berlinite: A mechanism for amorphization

Elastic instabilities have been used to explain the occurrence of amorphization in both {α}-quartz (SiO₂) and {α}-berlinite (AlPO₄). 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 PO₄ units in {α}-berlinite remained completely intact with only the aluminum changing coordination. This resulted in amorphization around the PO₄ 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.