High-pressure silica polymorphs as hardest known oxides

We report the results of an experimental study of the silica phases (stishovite and SiO2 with α-PbO2 like structure). These phases were synthesized in laser- and externally electrically heated Mao–Bell type diamond anvil cells (DACs) from α-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 α-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 polycrystalline silica phases are hardest known oxides and representatives of hardest known polycrystalline materials in general.