Volume measurement of MgO at high pressures and high temperatures

Abstract

The unit cell volume of periclase (MgO) has been measured under high pressures and high temperatures up to 10 GPa and 1400°C. The measurement was made by an in situ X ray diffraction technique with the DIA-type high multianvil high-pressure apparatus (SAM85) installed at the superconducting wiggler beamline at the National Synchrotron Light Source (NSLS, X17B). In order to avoid the effect of microscopic stress, a chunk of MgO sintered with a piston cylinder apparatus was used for the measurement. Special care was taken to obtain deviatoric-stress-free spectra; for the room temperature measurement, a liquid pressure medium was used, and in the high temperature experiments, the relaxation of deviatoric stress by the heating treatment was always checked. As a result, very precise unit cell volumes of MgO were determined using its six to eight diffraction peaks. MgO showed no structural distortion in the P-T conditions in this study. Fitting the experimental P-T-V data to the high-temperature Birch-Murnaghan equation of state yields K 0 = 153(3) GPa and (∂K T /∂T) p = -0.034(3) GPa/K, assuming a constant value of 4 for K' T,0 . The high temperature equation of state of MgO can be used as an experimental pressure standard in high P-T condition where NaCl scale is useless.