| Based on the first-principles computing method of density functional way and the plane-wave method, ZnO is studied using local density approximation. The first-principle method was adopted in this thesis to present the bull modulus, and equilibrium volume as well as lattice dynamic and transformation mechanism with regarding to different atomic configurations. From the difference between NaCl structure(RS), CsCl phase(B2), zinc-blende(ZB) and wurtzite(WZ), the total energy differences were calculated as a function of cell volume. This indicates the relative phase stabilities and possible phase transitions tendencies, which is consistent with the experimental observations and other previous calculations.Obviously, my focus is on the phase transition mechanism. In this paper we use two different theories to study the mechanism phase change of ZnO, one is soft mode theory, another is molecular dynamics simulation. By analyzing the results of calculated by the two methods, you can conclude that:(1) It is found that the transverse acoustic model of the phonon spectra of the wurtzite decreases with the increase of pressure. The imaginary frequencies emergences at M-point and N-point near 10 GPa. The pathway of phase transition from wurtzite to rocksalt structure is "tetragonal" rather than "hexagonal" in the perspective of the soft-model theory through the analyses to eigenvector of the vibration. However, it is other phenomenon for B2 phase. With the increase of pressure, the imaginary frequencies of phonon decreases and vanishes when the pressure is 340 GPa, which illustrate that B2 structure only exists above 340 GPa. (2) Found that WZâ†'RS, ZBâ†'RS phase transition pressure was 33GPa,27GPa, its value in the 30-35GPa,25-30GPa between. And then, the calculation the enthalpies of wurtzite, zinc-blend and rocksalt show that the phase transition pressures for both wurtzite and zinc-blend to rocksalt are 10 GPa, in which the pressure of wurtzite phase transition are very close to the experimental datas and theoretical values, while the value of zinc-blend is greater than experimental values and less than other theoretical values; in the interactions of pressure between wurtzite and zinc-blend individually, the phase transition is impossible because the changing trends of their enthalpies with the increase of pressure are alike. But phase transition is possible in the corporate interactions between the pressure and temperature. (3) The pressure-induced phase transitions of wurtzite ZnO is studied via a constant pressure ab initio molecular dynamics method. A first-order phase transition from wurtzite to rocksalt phase at 30-35 GPa was simulated. The transformation consists of three successive processes. Firstly, the wurtzite crystal transforms into a hexagonal structure (space group P63/mmc) with a compression along the c axis. Secondly, the hexagonal phase becomes unstable with respect to shear deformation and transforms into an orthorhombic intermediate state within Fmmm symmetry, and then into a rocksalt state. It is the hexagonal paths not the tetragonal paths for WZ-to-RS phase transformation at high pressure in our molecular dynamics simulations process. (4) Through above conclusions can be drawn that at low pressure WZâ†'RS phase transformation path is the "tetragonal" path, high pressure WZâ†'RS as a "hexagonal" path, which is the same with the conclusion of others calculation. (5) Ab initio MD method has been employed to investigate the phase transition from zinc blende-to-rocksalt structure in ZnO under high pressure. It was shown that at 27.0 GPa the ZB-to-RS phase transition in ZnO occurs via a tetragonal (space group: I-4m2) and monoclinic (space group:Cm) transition state. Therefore, the ZB-to-RS phase transition of ZnO obtained is cubic-tetragonal-monoclinic mechanism. This mechanism is similar compare with the ZB-to-RS of phase mechanism of SiC predicted by both ab initio and classical MD simulations, that of ZnS, and that of a model ionic system in MD simulations. |
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