In situ Characterization of Phase Transitions in Cristobalite under high Pressure by Raman Spectroscopy and X-Ray Diffraction.

Abstract We have studied, in situ, the structure and optical properties of cristobalite under non-hydrostatic pressures of up to 61 GPa by means of micro-Raman spectroscopy and X-ray powder diffraction. The starting material, the α-cristobalite phase of SiO 2 (C-I), was formed after annealing of silica sol-gel glass at 1500°C for 15 min. Mao-Bell and membrane-type diamond anvil cells were used for generation of the high pressure. On increasing the pressure, four polymorphs were found: C-I α-cristobalite up to 6 GPa, C-II in the pressure range 0.2–14 GPa, C-III from 14 to 35 GPa, and C-IV above 35 GPa, and no new phase was observed up to 61 GPa. The high-pressure phase C-IV is crystalline and quenchable. A monoclinically distorted α-PbO 2 -type structure provides the best fit to the X-ray powder pattern of the recovered phase C-IV.