X-ray diffraction from stishovite under nonhydrostatic compression to 70 GPa: Strength and elasticity across the tetragonal → orthorhombic transition

The tetragonal phase of silica (stishovite) was synthesized at high pressure and temperature in a laser-heated diamond anvil cell. Nonhydrostatic pressure condition was produced by pressurizing the sample without any pressure transmitting medium. The tetragonal -> orthorhombic transition could be detected from the X-ray diffraction patterns at similar to 40 GPa. In contrast, the orthorhombic phase has been reported to occur only above similar to 60 GPa in an earlier experiments under hydrostatic pressure. However, the transition pressures derived from the square of the symmetry-breaking strain versus pressure data in the two cases differ only marginally, the values being 44(8) GPa and 49(2) GPa under nonhydrostatic and hydrostatic compressions, respectively. We combine the d-spacings measured under nonhydrostatic and hydrostatic compressions to derive a parameter Q(hkl) that contains the information on differential stress t (a measure of compressive strength) and single-crystal elasticity. The compressive strengths derived from the average value of Q(hkl) and line-width analysis agree well. It increases from similar to 4 GPa at 20 GPa to similar to 8 GPa at 40 GPa and decreases as the transition pressure is approached. In the orthorhombic phase, t increases with pressure monotonically. The mean crystallite size of the sample decreases from similar to 5000 angstrom to similar to 1000 angstrom as the pressure is increased from 20 GPa to 45 GPa and remains nearly unchanged between 45 GPa and 70 GPa. The single-crystal elastic moduli derived from the X-ray diffraction data indicate that (C-11 - C-12) decreases rapidly as the transition pressure is approached. Line-width analysis of the diffraction lines suggests that near-hydrostatic pressure condition is achieved by laser annealing of the compressed sample. (c) 2012 Elsevier B.V. All rights reserved.