PRESSURE-INDUCED AMORPHIZATION, ELASTIC INSTABILITY, AND SOFT MODES IN ALPHA-QUARTZ

Pressure-induced amorphization has been observed in a number of oxide crystals, including alpha-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.