Quasi-hydrostatic compression of magnesium oxide to 52 GPa: Implications for the pressure-volume-temperature equation of state

Room temperature static compression of MgO (periclase) was performed under nearly hydrostatic conditions using energy dispersive synchrotron X-ray diffraction in a diamond anvil cell with methanol-ethanol (to 10 GPa) or helium (to 52 GPa) as a pressure-transmitting medium. Highly precise cell parameters were determined with an average relative standard deviation <<Delta>a/a> = 0.0003 over all the experimental pressure range. Fixing the bulk modulus K-0T = 160.2 GPa, a fit of the data to the third-order Birch-Murnaghan equation of state yields: V-0 = 74.71 +/- 0.01 Angstrom (3), (partial derivativeK(0T)/partial derivativeP)(T) = 3.99 +/- 0.01. A fit of different P-V-T datasets, ranging to 53 GPa and 2500 K, to a Birch-Murnghhan-Debye thermal equation of state constrained the Gruneisen parameter gamma (0) = 1.49 +/- 0.03, but not its volume dependence q, which was constrained to 1.65 +/- 0.4 by thermodynamic theory. A model based on a constant value of q cannot explain the ultrahigh pressure (P = 174-203 GPa) shock compression data. We developed a model in which q decreases with compression from 1.65 at 0.1 MPa to 0.01 at 200 GPa. This model, within the framework of the Mie-Gruneisen-Debye assumptions, satisfactorily describes the low-pressure static data (<<Delta>V/V> = 0,4% to 53 GPa) and the high-pressure Hugoniot data (<<Delta>V/V> < 1% to 203 GPa). Average values of the thermal expansion coefficient a range between 14.1 +/- 2.8 and 16.3 +/- 2.7 x 10(-6) K-1 at P = 174-203 GPa. The pressure dependence of the melting temperature yields an initial pressure derivative <partial derivative>T-m/partial derivativeP = 98 K/GPa. Our analysis shows that it is possible to develop a simple model of the volume dependence of the Gruneisen parameter that can successfully describe the P-V-T equation of state of MgO from ambient conditions to 203 GPa and 3663 K.