A class of new high-pressure silica polymorphs

We synthesised a number of new metastable silica modifications in the electrically heated diamond anvil cells (DACs) at pressures over 100 GPa and temperatures over 1200 K. Our studies reveal the existence of a new class of silica phases with different degree of the ordering of silicon atoms. Silica polymorphs with octahedrally co-ordinated silicon known so far (stishovite, fully disordered phase with niccolite-type structure, alpha-PbO2-type, and monoclinic (P2(1)/c space group) post-quartz phase) are members of this class. The details of the silicon distribution among free positions in the oxygen network strongly depend on the starting material and the conditions of high-pressure high-temperature treatment of silica, but a general trend is that at pressures above 40-45 GPa most of the silicon atoms are placed in octahedral positions. Our structural model provides a key for an explanation of the mysterious behaviour of silica, known for more than 30 years, when at the same pressure and temperature range different phases were synthesised. (C) 2004 Elsevier B.V. All rights reserved.