CALCULATIONS OF PRESSURE-INDUCED PHASE-TRANSITIONS IN SILICA

The crystal structures and energies of the quartz, stishovite and CaCl 2 phases of silica were calculated as a function of pressure with the polarization-included electron gas (PEG) model. The calculated structu res are in good agreement with experiment throughout the pressure rang e in which experimental results exist. The calculated pressure of the quartz to stishovite phase transition is about 3.5 GPa, as compared to an experimental value of 5.5 GPa. The calculated pressure of the stis hovite to CaCl2 structure phase transition is slightly less than 200 G Pa, higher than the value suggested by experiments of about 100 GPa. T he compressibility of quartz, the quartz to stishovite transition pres sure, and the stishovite to CaCl2 structure transition pressure are al l calculated more accurately with the PEG model, which allows the ions to distort from spherical symmetry, than with the modified electron g as model, which constrains the ions to be spherical.