DYNAMICAL INSTABILITIES IN ALPHA-QUARTZ AND ALPHA-BERLINITE - A MECHANISM FOR AMORPHIZATION

Elastic instabilities have been used to explain the occurrence of amor phization in both alpha-quartz (SiO2) and alpha-berlinite (AlPO4). How ever, there is a dynamical instability at (1/3,1/3,0) in the Brillouin zone preceding the elastic instability for both structures which impl ies that distortion of a 3X3X1 supercell will form a more stable cryst al structure. Simulations of distorted supercells of size 3X3X1 result ed in the collapse of the structures below the elastic instability pre ssures for both alpha-quartz and alpha-berlinite. A subtle difference between them was that the eigenvectors for phosphorus were smaller tha n those of silicon or aluminum and on ''amorphization'' the PO4 units in alpha-berlinite remained completely intact with only the aluminum c hanging coordination. This resulted in amorphization around the PO4 un its with no P-O-P links formed and a return to a crystalline phase on the release of pressure for alpha-berlinite, which was not found in th e simulations of alpha-quartz.