All-electron and pseudopotential study of MgO: Equation of state, anharmonicity, and stability

We have studied the high-pressure behavior of periclase (MgO) using density functional simulations within the generalized gradient approximation. The static and thermal (P-V-T) equation of state, B1-B2 transition pressure, elastic constants, Gruneisen parameter, and the intrinsic anharmonic parameters were calculated from static and ab initio molecular dynamics simulations. The simulations were performed using the projector augmented-wave and pseudopotential methods with different descriptions of the Mg atom ("small core" and "large core"). The errors of large-core pseudopotentials increase with pressure and are mainly due to the overlap between the Mg semicore (2p) orbitals and the valence orbitals, both of the same Mg atom and of the neighboring O atoms, rather than core deformation or core-core overlap effects. In agreement with previous works, we find that MgO remains in the B1 ("NaCl") structure at all pressures existing within the Earth, and transforms into the CsCl-type structure at 509 GPa. Direct ab initio calculations avoid the simplifying assumptions inherent to many empirical treatments of thermoelasticity and allowed us to assess some of the common assumptions. We present a detailed qualitative analysis of the effects of intrinsic anharmonicity and analyze the validity of the Mie-Gruneisen approximation at high temperatures.