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 pressuretransmitting medium. Highly precise cell parameters were determined with an average relative standard deviation = 0.0003 over all the experimental pressure range. Fixing the bulk modulus K_OT= 160.2 GPa, a fit of the data to the third-order BirchMurnaghan equation of state yields: V₀ = 74.71 ± 0.01 ų,(∂K_0T/∂P)_r=3.99±0.01. Afit of different P-V-T datasets, ranging to 53 GPa and 2500 K, to a Birch-Murnaghan-Debye thermal equation of state constrained the Grüneisen parameter γ₀ = 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 = 0.4% to 53 GPa) and the high-pressure Hugoniot data (1% to 203 GPa). Average values of the thermal expansion coefficient α range between 14.1 ± 2.8 and 16.3 ± 2.7 × 10^-6 K^-1 P = 174-203 GPa. The pressure dependence of the melting temperature yields an initial pressure derivative ∂T_m/∂P = 98 K/GPa. Our analysis shows that it is possible to develop a simple model of the volume dependence of the Grüneisen parameter that can successfully describe the P-V-T equation of state of MgO from ambient conditions to 203 GPa and 3663 K.