THE USE OF QUARTZ AS AN INTERNAL-PRESSURE STANDARD IN HIGH-PRESSURE CRYSTALLOGRAPHY

The unit-cell parameters of quartz, SiO2, have been determined by sing le-crystal diffraction at 22 pressures to a maximum pressure of 8.9 GP a (at room temperature) with an average precision of I part in 9000. P ressure was determined by the measurement of the unit-cell volume of C aF2 fluorite included in the diamond-anvil pressure cell. The variatio n of quartz unit-cell parameters with pressure is described by: a-4.91 300 (11)= -0.0468 (2) P + 0.00256 (7) P-2 -0.000094 (6) P-3, c -5.4048 2 (17)= -0.03851 (2) P+0.00305 (7) P-2 -0.000121 (6) P-3, where P is i n GPa and the cell parameters are in angstroms. The volume-pressure da ta of quartz are described by a Birch-Murnaghan third-order equation o f state with parameters V-0=112.981 (2)a(3) K(T0)037.12(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 thi rd-order equation. The use of oriented quartz single crystals is propo sed 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 prec ision in pressure measurement of 0.009 GPa at 9 GPa.