Comparative Investigations Of Thermodynamic Properties Of MgO With Shell-model MD Simulations At High Pressures

As one of the major Earth-forming minerals, MgO is stable in rock-salt structure with the pressure up to 227 GPa. The knowledge of the high-pressure thermodynamc properties of MgO is most important for interpreting the seismically observed velocity variations in the Earth's mantle and for constructing reliable mineralogical models of the Earth's interior.MD simulations have been performed to investigate the P-V-T equation of state of MgO under high pressures and temperatures using the reliable shell-model (SM) potentials. In order to take account of non-central forces in crystals, the breathing-shell-model (BSM) is also introduced in MD simulation. Simulated volume compression and the equation of state for MgO are in good agreement with experimental values at 300 K.For isothermal compressions, at higher temperatures and under lower pressures, the volume is greatly affected by the pressure. The volume varies quickly as the temperature rises under lower pressures, and the T-V relations are nearly linear under higher pressures (about 100 GPa) for the isobaric curves. The properties of MgO are summarized in the pressure range of 0-200 GPa and the temperature up to 3500 K.The isothermal bulk modulus of MgO is simulated by MD method with SM and BSM interionic potentials. The isothermal bulk modulus of MgO dependence of the compression ratio and the pressure have been obtained from MD runs at 300 and 2000 K, and compared with the available theoretical results. Compared with SM potential of Stoneham and Sangster, the MD results with potential of Lewis and Catlow are found to be in good agreement with the studies based on ab initio calculations, and the results obtained using BSM potential are more compressible. At an extended pressure and temperature ranges, the isothermal bulk modulus has also been predicted. It can be found that the isothermal bulk modulus decreases with increasing temperatures and increases with increasing pressure.The thermal expansivity of the rock-salt phase of MgO have been investigated in the pressure range of 0 ~200 GPa and in the temperature range of 300 ~3500 K using MD method.The fundamental conclusions about the material have been obtained. Note that the bulk thermal expansion coefficient decreases with increasing pressures, and increases with elevating temperatures at lower pressures. The influence of temperature on thermal expansion at high pressures is very small and may be neglected at above 50 GPa pressure. Thus, the volume thermal expansivity of MgO at high pressures is a weak function of temperature. The thermodynamic parameters mentioned above are of fundamental importance in condensed matter physics.