RAMAN-SPECTROSCOPIC STUDY OF HIGH-PRESSURE PHASE-TRANSITIONS IN CRISTOBALITE

The pressure dependence of the cristobalite Raman spectrum has been in vestigated to 22 GPa at room temperature, using single-crystal Raman s pectroscopy with a diamond-anvil cell. We observe a rapid, first-order phase transition on increasing pressure, consistent with the cristoba lite I <--> II transition revealed in previous x-ray diffraction exper iments. The phase transition has been bracketed at 1.2 +/- 0.1 GPa on increasing pressure and 0.2 +/- 0.1 GPa on decreasing pressure. The pr essure shifts of 11 Raman bands in the high-pressure phase (cristobali te II) have been measured. Evidence for an unusual hybridization of mo des at 490-500 cm-1 is found. Changes in the Raman spectra also reveal an additional phase transition to a new phase at P almost-equal-to 11 GPa, which remains to be fully characterized.