New Polymorph of SiO₂ Formed under Quasi-Hydrostatic Compression of Cristobalite

Abstract

High pressure in situ x-ray observations of silica (cristobalite) clarified very large effect of non-hydrostaticity on the crystal structure of high pressure polymorph and the apparent compressibility. Under quasi-hydrostatic compression, cristobalite transformed to a new polymorph, which gives very similar diffraction profile to that of stishovite, while under strongly non-hydrostatic compression, a completely different phase named "X-I" was formed. Very different compression curves of cristobalite were observed, depending on the hydrostaticity of applied pressure, which is well explained using the elastic theory.