THE PRESSURE BEHAVIOR OF ALPHA-CRISTOBALITE

Structural and volume compressibility data for two a cristobalite samp les were determined by single-crystal X-ray diffraction methods at pre ssures up to approximately 1.6 GPa. As with the other silica polymorph s that have been studied at high pressure, the change of the Si-O-Si a ngle is correlated with the volume compressibility. The Si-O bond leng ths and the O-Si-O angles remain essentially unchanged. The bulk modul us was determined to be 11.5(7) GPa with a pressure derivative of 9(2) . Both crystals underwent reversible structural phase transitions in t he pressure interval 1.18-1.60 GPa.