Nature 336, 670 - 672 (15 December 1988); doi:10.1038/336670a0

Theoretical evidence for a new ultra-high-pressure phase of SiO²

Key Taeck Park^*, Kiyoyuki Terakura^* & Yoshito Matsui^†

^*Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan
^†Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori-ken 682-02, Japan

A phase of silica denser than stishovite (SiO₂-rutile) has been searched for intensively. Following the suggestions by APtshuler et al.¹ and Simakov et al. ², SiO₂-fluorite was once regarded as a candidate for such a high-pressure phase. Subsequent theoretical studies^3,4 revealed, however, that SiO₂-fluorite would have at most about the same density as SiO₂-rutile and that it would be dynamically unstable at pressures below 170 GPa (ref. 4). Here we propose an alternative hypothetical polymorph of silica with a modified fluorite (Pa&3macr;) structure as a possible high-pressure phase. SiO₂-Pa&3macr; would have a density of 4.46 gcm^-3 (6% denser than SiO₂-rutile at normal pressures) and should become more stable than SiO₂-rutile at pressures of >=60 GPa. These results also suggest that MgSiO₃-perovskite, which is widely accepted as the major constituent within the Earth's deep interior, may be unstable at very high pressure.

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