HIGH-PRESSURE BEHAVIOR OF SILICA AND ITS ANOLOGS Kathleen J. Kingma, Rosemary E. Gerald Pacalo, and Paul F. McMillan, Arizona State University, Materials Research Group in High-Pressure Synthesis, Department of Chemistry and Biochemistry, Box 871604, Tempe, AZ 85287-1604

The response of silica to high pressure has been studied extensively. Diamond-cell experiments have revealed that SiO₂ quartz exhibits a rich and complicated polymorphism, involving an amorphous phase, a quartzlike phase, and a stishovite-like crystalline phase that persists to above 200 GPa. Pressurization of SiO₂ cristobalite indicates several crystallinecrystalline phase transformations which occur below 40 GPa, and amorphization has been reported near 30 GPa. We have recently prepared phosphorous oxynitride (PON), a material isoelectronic with SiO₂, having acristobalite-like P4 structure. Diamond-cell compression to 70 GPa without a medium shows gradual transformation of the P4 phase to a cristobalite I42d structure by 20 GPa. Under continued nonhydrostatic compression, the I42d phase persists to 70 GPa. Unlike the corresponding SiO₂ framework structures, there is no evidence for pressure-induced amorphization.