The Study Of Computer Simulation For The Equation Of State Of ZnO And MgO

This dissertation gives a brief introduction of the developed history and latest investigation about the equation of state and the thermodynamic properties of substance. The focus is on the behavior of the important component of the Earth's lower mantle (660 - 2890 km depth) like ZnO and MgO. The Buckingham potential has been employed to simulate the thermodynamic properties of ZnO with rock-salt structure at high pressures and temperatures using molecular dynamics (MD) method. Shell model molecular dynamics (MD) method is used to predict the structure and thermodynamic properties of MgO at high temperatures and high pressures using the Stoneham-Sangster and Lewis-Catlow potentials, respectively. In order to account for the observed large departures from the Cauchy relation of the elastic constants of the MgO, the breathing shell model (BSM) is also introduced in MD simulation, in which the repulsive radii of oxygen ions are allowed to deform isotropically under the effects of other ions in the crystal. These properties including the expansivity, constant-pressure heat capacity, isothermal bulk modulus are calculated in a wide range of temperatures (300-3000 K) and pressures (0-150 GPa). The obtained structural and thermodynamic parameters are compared with the available experimental data and other theoretical results. Compared with Stoneham-Sangster and Lewis-Catlow potentials, the MD simulation with BSM is very successful in reproducing accurately the measured volumes of MgO and the results are more compressible. Meanwhile, some thermodynamic parameters have been predicted at elevated temperatures and high pressures. The detailed knowledge of thermodynamic behavior of the major Earth-forming mineral - periclase (MgO) at extreme P-T conditions are of fundamental importance to our understanding of the Earth's lower mantle and the history of the Earth's formation. The equation of state, isothermal bulk modulus and thermal expansion coefficient of the rock-salt phase of ZnO have been predicted in the pressure range of 0～50GPa and in the temperature range of 273～2273K. The heat capacity of rock-salt phase of ZnO has also been calculated. The fundamental conclusions about the two materials have been obtained.In this paper, we firstly investigated the equation of state, isothermal bulk modulus and thermal expansion coefficient of the rock-salt phase of ZnO in the pressure range of 0～50GPa and in the temperature range of 273～2273K using MD simulations and found that the isothermal bulk modulus decreases with elevating temperatures and increases with increasing compression ratio, constant-pressure heat capacity increases with elevating temperatures and decreases with increasing pressures. Compared with lower pressures, the influence of temperature is small for constant-pressure heat capacity at high pressures. Note that the bulk thermal expansion coefficient decreases with increasing pressures; and increases with elevating temperatures at lower pressures. Meanwhile, some thermodynamic parameters and the equation of state of MgO are simulated and predicted in the pressure range of 0～150GPa and in the temperature range of 300～3000K using MD method with three different potential model. Comparing the results of the equation of state and thermodynamic properties of MgO with molecular dynamics simulations, we found that the MD simulation with BSM is very successful in reproducing accurately the properties of MgO and the results are more compressible than the results that obtained by MD with the other two potentials. The BSM-MD simulated 300K isotherms are in good agreement with experimental and theoretical results. This suggests that the BSM might be applied to calculate thermodynamic properties of MgO and gives us a solid ground for an application under high pressures. The detailed knowledge of thermodynamic behavior of the major Earth-forming mineral - periclase (MgO) at extreme P-T conditions are of fundamental importance to our understanding of the Earth's lower mantle and the history of the Earth's formation.