THE PYRITE-TYPE NEW HIGH-PRESSURE FORM OF SILICA

Silica (SiO₂) exhibits extensive polymorphism at elevated pressures. Previous theoretical calculations predicted the phase transition from α-PbO₂-type to pyrite-type structure around 200 GPa. However, no experimental studies of silica have been made at such multimegabar pressures. We performed in-situ X-ray diffraction measurements of silica up to 291 GPa using the laser-heated diamond-anvil cell (LHDAC) techniques at the synchrotron facility of SPring-8. Results show that the pyrite-type high-pressure form is stable above 268 GPa and 1800 K. Rietveld analysis showed that the pyrite-type silica has six Si-O bonds with 1.608 Å distance and additional two interpolyhedral Si-O bonds with 2.372 Å distance at 271 GPa and 300 K. Such interpolyhedral Si-O distance in pyrite-type structure is much shorter than that in α-PbO₂-type structure. The increase in coordination number of Si from 6 to 6+2 results in a large increase in density. The density of pyrite-type silica is 6.576(1) g/cm³ at 271 GPa and 300 K, which is larger by 4.7% than that of α-PbO₂-type phase when compared at equivalent pressure. The new phase may be an important constituent in a rocky core of Uranus and Neptune. It may be also included in the terrestrial extrasolor planets.