Phase Transition And Elastic Properties Of Several Materials Under High Pressure Calculated By First Principle Method

High pressure physics is a subject that studies the physical behavior of material under high pressures, and is becoming more and more important. Materials at high pressures occur at the centers of planets and in stars and in both natural and man-made explosions. The properties of the material under high pressure may be very different from those under normal conditions. One of the interesting phenomena is that under certain pressure a sudden change in the arrangement of the atoms may occur, i.e., a structural phase transition. Studying the phase transition of material has very important effect in phase diagram and properties. In these properties, the elastic property is a very important part. The elastic constants of solids give important information on their mechanical and dynamical properties. These parameters provide a link between the mechanical and dynamic behavior of crystals, and may be used as a means of probing the inter-atomic forces. In particular, they provide information on the stability and stiffness of materials. The thesis is made of six chapters.In Chapter One, we introduce the principle of high-pressure experiment and the development of high pressure phase transition study in calculation. In section "the principle of high pressure experiment", we mainly introduce the principle of the diamond anvil cell (DAC) and the measurement of pressure; in section "the development of high pressure phase transition study in calculation", we introduce the development of single atom materials, AB-type materials and complex compounds, and the advantage and disadvantage of two method: total energy calculation and molecular dynamics method.In Chapter Two, we give introduction to the first principle molecular dynamics based on pseudo-potential and plane wave functions. How to use program CPMD is also mentioned.In Chapter Three, we study the high pressure phase transition of CS₂ . CS₂ and CO₂ are compounds in same group. The phase transition of CS₂ was first investigated at 1941. Since then, many people had studied this material, and then a phase diagram was obtained. At recent times, people found that under high pressure and high temper- ature, CO₂ can change to a structure like SiO₂. Then we got an idea that CS₂ may be have the same property at high pressure and high temperature and this was never see in the former studies. By using molecular dynamic combined total energy calculation method, we studied the phase transition of CS₂ at room temperature and high pressure. We predict a phase transition from Cmca toβ-quartz at about 10.6 GPa. And when temperature is about 1000 K and pressure is 20 GPa, the dissociation of CS₂ into its constituent elements C and S were observed.In Chapter Four, the high pressure phase transition and elastic properties changed with pressure of CdO and MgS were investiged. CdO is an IIB-VIA semiconductor. The phase transition of CdO under high pressure is from RS to CsCl at about 90.4 GPa in experiment. We studied the phase transition of CdO by the total energy calculation method using a simple Buckingham type potential. The same prediction was observed at a pressure about 95 GPa. This result agrees well with experiment. The elastic properties changed with pressure under these two structures were also calculated.The phase transition of MgS was studied by others, predicted a phase transition form RS to CsCl at above 160 GPa. Also, we have known that the ZB MgS has been synthesized. We studied the elastic properties of RS and ZB MgS changed with pressure by using first principle method. The calculation shows that the two structures essentially have the same enthalpy at equilibrium state. This can explain why the ZB MgS can be obtained at experiment. We also found that the ZB MgS is very unstable, and would never exist at above 5 GPa. Except for the elastic property, the Debye temperature and sound velocity were also calculated in our calculation.In Chapter Five, we studied the elastic properties of singlewall ZnO and ZnS nanotubes. ZnO and ZnS nanotubes are one of the materials that are most investigated in recent times. Now, there are many methods to synthesize these two nanotubes. But the investigation on structural and elastic properties is few. We investigated the structural and elastic properties of these two nanotubes. They all have the similar behavior as carbon nanotube. But the Young modulus of ZnS nanotube is much smaller than ZnO nanotube.In Chapter Six, we summarize the contents of former chapters.