MGO AT HIGH-TEMPERATURES AND PRESSURES - SHELL-MODEL LATTICE-DYNAMICS AND MOLECULAR-DYNAMICS

MgO at high temperatures and pressures is studied by means of lattice dynamics in the quasi-harmonic approximation, and by molecular-dynamics simulation. The shell model is used to represent the polarizability of the oxygen ion. The two methods give very similar predictions for thermal expansion. However, the lattice dynamics fails at temperatures above 1500 K at atmospheric pressure, though not at high pressure (100 GPa). The predicted thermal expansion, using the Stoneham-Sangster and Lewis-Catlow potentials, is less than that observed experimentally. Predicted volumes at high pressure are too large. Shell-model molecular-dynamics simulation can be used to study the dynamics at temperatures close to the melting point. A small anharmonic shift of vibrational frequencies is observed.