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 ∼40 GPa. In contrast, the orthorhombic phase has been reported to occur only above ∼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 ∼4 GPa at 20 GPa 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 ∼5000 Å to ∼1000 Å 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.