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.