The Use of Quartz as an Internal Pressure Standard in High-Pressure Crystallography

The unit-cell parameters of quartz, SiO₂, have been determined by single-crystal diffraction at 22 pressures to a maximum pressure of 8.9 GPa (at room temperature) with an average precision of 1 part in 9000. Pressure was determined by the measurement of the unit-cell volume of CaF₂ fluorite included in the diamond-anvil pressure cell. The variation of quartz unit-cell parameters with pressure is described by: a -4.91300 (11) = -0.0468 (2) P + 0.00256 (7) P² - 0.000094 (6) P³, c - 5.40482 (17) = - 0.03851 (2) P + 0.00305 (7) P² - 0.000121 (6) P³, where P is in GPa and the cell parameters are in ångstroms. The volume-pressure data of quartz are described by a Birch-Murnaghan third-order equation of state with parameters V₀ = 112.981 (2) å³, K_T0 = 37.12 (9) GPa and K' = 5.99 (4). Refinement of K'' in a fourth-order equation of state yielded a value not significantly different from the value implied by the third-order equation. The use of oriented quartz single crystals is proposed as an improved internal pressure standard for high-pressure single-crystal diffraction experiments in diamond-anvil cells. A measurement precision of 1 part in 10 000 in the volume of quartz leads to a precision in pressure measurement of 0.009 GPa at 9 GPa.