In situ characterization of phase transitions in cristobalite under high pressure by Raman spectroscopy and X-ray diffraction

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 (alpha -cristobalite phase of SiO2 (C-I), was formed after annealing of silica sol-gel glass at 1500 degreesC 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 alpha -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 alpha -PbO2-type structure provides the best fit to the X-ray powder pattern of the recovered phase C-IV. (C) 2001 Elsevier Science BY All rights reserved.