PHASE-DIAGRAM AND PROPERTIES OF SILICA - LATTICE AND MOLECULAR-DYNAMICS STUDY

Properties and a phase diagram of silica were examined in a broad pres sure-temperature range using lattice and molecular dynamics methods an d a previously developed interatomic potential (Belonozhko and Dubrovi nsky, 1995). Good agreement was reached between the theoretical and ex perimental data on volumetric, caloric, and spectral properties. Phase stability relations such as quartz-coesite, coesite-stishovite, and s tishovite-melt can be reasonably well reproduced. At pressures above 8 3 GPa stishovite transforms into a CaCl2-like phase, and at pressures higher than 93 GPa and 110 GPa, according to lattice and molecular dyn amics calculations, respectively, a new phase with a Pnc2 structure, t ermed SBAD, becomes stable. Our analysis of simulated X-ray diffractio n patterns of various hypothetical poststishovite phases shows that th is new phase should always be considered in the treatment of high-pres sure experiments involving silica. The appearance of this new phase is consistent with seismic observations. Previous high-pressure experime ntal observations of poststishovite phases can be explained either by the appearance of the SBAD phase (Tsuchida and Yagi, 1989) or by a sma ll deviation from hydrostatic conditions (Kingma et al., 1995).