Elastic Instabilities and Amorphization of Crystalline Silica Under Pressure

Abstract

ABSTRACTSolid state amorphization can occur when a crystalline phase is compressed at a sufficiently low temperature to inhibit kinetically the transformation to a stable high pressure crystalline phase. An example of such a vitrification transformation occurs in α-quartz, the most stable phase of SiO2 at standard temperature and pressure conditions. Under pressure at room temperature α-quartz gradually transforms to an amorphous form in the range of 25-30 GPa. The driving force for this amorphization is not clear, and speculation has centered on mechanical instabilities of the quartz crystal under pressure. The elastic properties of α-quartz are studied as a function of pressure using both classical interatomic potentials, and ab initio pseudopotentials. In both cases, we find that the α-quartz structure becomes mechanically unstable at about 30GPa. This finding supports a picture in which the amorphization of quartz is triggered by the onset of a lattice shear-instability. The microscopic origin of this elastic softening is intimately related to the presence of an oxygen close-packed cubic arrangement in the quartz high pressure structure.