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

Abstract

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 ∼40GPa. In contrast, the orthorhombic phase has been reported to occur only above ∼60GPa 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 ∼4GPa at 20 GPa to ∼8GPa at 40GPa 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 ∼5000Å to ∼1000Å as the pressure is increased from 20GPa to 45GPa and remains nearly unchanged between 45GPa and 70GPa. The single-crystal elastic moduli derived from the X-ray diffraction data indicate that (C11-C12) 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.