Fundamental properties and phase stability of B1 and B2 phases of MgO over a wide range of pressures and temperatures: A first-principles study

Accurate determination of the phase diagram of MgO is proved to be a challenging task, mainly because of the very high B1–B2 transition pressures (P_t) and melting temperatures. Only very recently experimental observations of the B1–B2 and solid-liquid transitions of MgO have been achieved. In spite of the extensive experimental and theoretical studies, a satisfactory agreement about the phase diagram of MgO has not yet been reached. In this work, we will thoroughly investigate the various factors that may affect the calculated B1–B2 phase boundary, using first-principle techniques. The structural, vibrational and thermal properties of the B1 and B2 phases of MgO are also investigated. It has been found that P_t is quite sensitive to the set of pseudopotentials (PPs) used in the PP plane-wave approach, and volume (or pressure, P) range over which the Helmholtz free energy is directly calculated. An interplay between such a P range and the anharmonic effects is demonstrated, which allows (by a suitable choice of the P range) for a high reduction in these effects (i.e., highly extends the validity of the temperature range of the quasiharmonic approximation). The very recently proposed optimized norm-conserving Vanderbilt (ONCV) PPs are found to yield results in very good agreement with those obtained by the all-electron projected augmented wave calculations, contrary to other norm-conserving PPs. As for the exchange-correlation energy, the local density approximation is not only found to provide a good description of P_t of the above transition, but also its temperature dependence. The obtained results are discussed in comparison with the most recent theoretical results and experimental data. In particular, our results suggest that the estimated temperature of the B1–B2 transition directly observed by using the laser-driven ramp loading technique is highly exaggerated.