A NEW HIGH PRESSURE PHASE OF SILICA. R. Ahuja, O. Eriksson, J.M. Wills¹, and B. Johansson, Condensed Matter Theory Group, Department of Physics, Uppsala University, Box 530, S-751 21, Uppsala, Sweden, ¹Center for Materials Science and Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

The search for structural phase transitions in silica (SiO₂) at elevated pressures is of special interest in earth science. This is so since SiO₂ accounts for 50% in the chondritic model of the earths composition. The abundance of silica in the Earth's composition makes the high pressure structural properties of this material especially interesting from an earth science point of view. Based on the first principles full-potential linear muffin tin orbital calculations, we show that a new silica polymorph, named SBAD, becomes stable at pressures of the Earth's lowermost mantle. This structure is intermediate between the [[alpha]]-PbO₂ and baddeleyite structures. We find the following sequence of phase transitions; stishovite CaCl₂ SBAD Pa3. The first transition takes place at 45 GPa, the second at 80 GPa and the third at 220 GPa. The transition at 45 GPa is in agreement with the recent experiment and calculation. So we conclude on the basis of the first principles calculations, that a structure which so far has not been considered, named SBAD, must be viewed as a strong candidate for silica in the pressure range ~ 80-220 GPa.