Pressure-induced amorphization, elastic instability, and soft modes in α-quartz

Abstract

Pressure-induced amorphization has been observed in a number of oxide crystals, including α-quartz. We have examined the properties of quartz as a function of pressure using classical interatomic potentials, and ab initio pseudopotentials. With both approaches we find that the quartz structure becomes mechanically unstable in the 20–35 GPa range. The instability is reflected in a soft zone-edge phonon mode which occurs in the same pressure range. We have analyzed the microscopic origin of the softenings, and relate them to the presence of an oxygen close-packed cubic arrangement in the quartz high-pressure structure. Within this close-packed arrangement, the silicon cations exhibit an instability between fourfold and sixfold coordination sites.