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

MgO at high temperatures and pressures is studied by means of lattice dynamics in the quasi-harmonic approximation, and by molecular-dynamic s simulation. The shell model is used to represent the polarizability of the oxygen ion. The two methods give very similar predictions for t hermal 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 an d Lewis-Catlow potentials, is less than that observed experimentally. Predicted volumes at high pressure are too large. Shell-model molecula r-dynamics simulation can be used to study the dynamics at temperature s close to the melting point. A small anharmonic shift of vibrational frequencies is observed.