High-pressure silica polymorphs as hardest known oxides

We report the results of an experimental study of the silica phases (stisho vite and SiO2 with alpha -PbO2 like structure). These phases were synthesiz ed in laser- and externally electrically heated Mao-Bell type diamond anvil cells (DACs) from a-cristobalite as a starting material at pressures above 80 GPa and temperatures above 1200 K. We measured the hardness (H) of both phases and found that the value for the alpha -PbO2-like silica, H=31+/-3 GPa, is very close to that of stishovite, H=32+/-2 GPa, for polycrystalline material obtained in electrically heated DACs. Thus, these two polycrystal line silica phases are hardest known oxides and representatives of hardest known polycrystalline materials in general. (C) 2001 Elsevier Science B.V. All rights reserved.