Molecular Dynamics Study For The Melting And Thermodynamic Properties Of ZnO And GaN At High Pressures And Temperatures

This dissertation gives a brief introduction of the developed history and latest investigation about the thermodynamic properties of substance. The focus is on the behavior of the ZnO and GaN. The Buckingham potential has been employed to simulate the thermodynamic properties of ZnO with rock-salt structure and GaN with zinc-blende structure at high pressures and temperatures using molecular dynamics (MD) method. Firstly, The reliability of the present potential model has been verified. Secondly, the melting of ZnO with rock-salt structure, GaN with zinc-blende structure and the Earth-forming mineral, MgO, have been investigated in the pressure range of 0~150GPa. Finally, the isothermal bulk modulus and thermal expansion coefficient of the rock-salt phase of ZnO and the zinc-blende phase of GaN have been predicted in the pressure range of 0~150GPa and in the temperature range of 300-3000K. The heat capacity and Gruneisen parameter of zinc-blende phase of GaN have also been calculated. The fundamental conclusions about the two materials have been obtained.In this paper, we firstly investigated the melting of ZnO with zinc-blende structure at normal pressure using MD simulations and found superheating existed. The degree of superheating was 48% compared with experimental data 2248K. According to the result of superheating of ZnO with zinc-blende structure, we have modified that the room pressure melting temperatures of the rock-salt phase of ZnO and the zinc-blende phase of GaN. Meanwhile, the melting curve of the rock-salt phase of ZnO obtained by MD method is also given. The melting temperatures 2465K of the rock-salt phase of ZnO and 2327K of the zinc-blende phase of GaN have been obtained at normal pressure, respectively. The melting curve of the rock-salt phase of ZnO from MD calculations is in good agreement with the results obtained from Lindemann melting equation in the pressure below 7GPa. The extrapolated melting temperatures for MgO in the lower mantle are in good agreement with the results obtained from Wang's empirical model up to 100GPa.The isothermal bulk modulus and thermal expansivity of the rock-salt phase of ZnO have been investigated using MD method. The structural and thermodynamic properties of thezinc-blende phase of GaN including lattice constant, phase transition pressure (from the zinc-blende to the rock-salt structure), thermal expansion, isothermal bulk modulus, specific heat and Gruneisen parameter have also been investigated systematically. The calculated null compression isothermal bulk modulus and bulk thermal expansion coefficient of the rock-salt phase of ZnO, and isothermal bulk modulus, thermal expansion coefficient, heat capacity and Gruneisen parameter of the zinc-blende phase of GaN under normal state are compared with the available experimental data and other theoretical results. At an extended temperature and pressure ranges, the isothermal bulk modulus and bulk thermal expansion coefficient have also been predicted up to 3000K and 150GPa based on the reliable potential model. Note that the isothermal bulk modulus decreases with elevating temperatures and increases with increasing compression ratio. Bulk thermal expansion coefficient decreases with increasing pressures, and increases with elevating temperatures at lower pressures. Heat capacity of the zinc-blende phase of GaN increases with elevating temperatures, constant-volume heat capacity decreases with increasing volumes, and constant-pressure heat capacity decreases with increasing pressures. Compared with lower pressures, the influence of temperature is small for constant-pressure heat capacity at high pressures. Gruneisen parameter of the zinc-blende phase of GaN increases with increasing volumes. The thermodynamic parameters mentioned above are of fundamental importance in condensed matter physics.