Abstract
The effect of pressure up to 223 kbar on the lattice parameters and volume of synthetic stishovite was determined at room temperature by means of X ray diffraction techniques. The zero pressure isothermal ulk modulus was evaluated to be 3.44 ± 0.27 Mbar by means of the least squares fit of the volume data to the Birch-Murnaghan equation assuming the K0′ value in the range 2–7. The compression along the a axis (Δa/a0) is approximately 1.5 times as great as the compression along the c axis (Δc/c0): Δc/c0 = (0.66 ± 0,24) Δa/a0. Similar relationships have been observed for TiO2 and GeO2; both are tetragonal and isostructural with stishovite, ultrasonic techniques being used to measure the elastic constants. It is thus suggested that the effect of pressure on the c/a ratios for oxides with the rutile-type structure is predominantly determined by the geometry of the structure and is independent of the size of the cation. The anisotropic compression for stishovite can be explained in terms of a greater extent of overlapping of the electronic orbits parallel to the c axis than of the directions perpendicular to the c axis.
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