THE PRESSURE-INDUCED FERROELASTIC PHASE-TRANSITION OF SIO2 STISHOVITE

Using a newly developed accurate ab initio technique, we study the sta bility and the lattice dynamics of SiO2 polymorph stishovite as a func tion of pressure. We observe a ferroelastic phase transition to CaCl2 structure at 64 GPa, driven by a mechanical instability associated wit h a vanishing shear modulus. Other lattice instabilities due to phonon anomalies are theoretically excluded along with a proposed softening of the B1g phonon mode.