ELASTIC INSTABILITY IN ALPHA-QUARTZ UNDER PRESSURE

The elastic properties of a-quartz are studied as a function of pressure using both a classical interatomic potential, and a first-principles pseudopotential approach. In both cases, we find that the structure becomes mechanically unstable at about 30 GPa. Our results suggest that the amorphization of quartz observed experimentally in this pressure range is triggered by the onset of a lattice shear instability. We analyze the microscopic origin of the elastic softening, and relate it to the presence of an oxygen close-packed cubic arrangement in the quartz high-pressure structure. A change in the Si coordination is found to be intimately related to the shear instability.