Pressure-induced amorphization in crystalline silica: Soft phonon modes and shear instabilities in coesite

Quartz and closely related materials will transform under pressure from crystalline states to amorphous forms. Here we examine coesite, a high-pressure form of silica which also undergoes pressure induced amorphization. We find that coesite, like quartz, possesses a shear instability closely coupled to a zone-edge phonon softening at pressures comparable to the amorphization transformation. The commonality of these features strongly suggests that a coupling between a shear and a phonon soft mode plays an important role in pressure induced amorphization. This mechanism is similar to that observed in martensitic transformations. The densities for the phases produced at high pressures, as calculated from variable cell shape molecular dynamics, follow the experimental glassy region joining coesite to stishovite.