First-principles Study Of Silane At High Pressure And High Temperature

In the universe, Hydrogen, the lightest element, which is the classic problem of condensed matter research material, is considered as a superconductor at high pressure Recently. In experiment, solid hydrogen which is compressed to 342 GPa still remains well performance of insulator. Until " pre-compression" is put forward, the hydrides of group IV attract people’s attention. A large quantity of data from theoretical and experimental researching show that methane(CH4) has not appeared features of metal even the pressure is increased to 400 GPa, so methane has no superconductivity researching value. Therefore, theoretical and experimental studies of hydrogen-rich compounds IV group are focused on the Silane(SiH4) at high temperature and high pressure.Silane’s physical properties will be changed at high temperature and high pressure, such as volume decreasing and the total energy increasing. In particular, the volume compression could lead to a reduced distance of atoms or molecules. According to the quantum theory, these changes would lead to a reconfiguration of the electron density in spatial distribution, and ultimately lead to a property change of silane physically and chemically. It is very important scientific significance to explore silane’s structural changes(electronic structures, lattice parameter, phase transformation, etc.) and the influence on the macroscopic physical properties(including thermal, electrical, optical properties, etc.) of molecular decomposition and atomic reorganization. And it’s conducive to solve the metallic hydrogen and the high temperature superconducting problems in the modern natural science and engineering technology. Currently, there are still strong controversies about the influence of temperature, pressure, volume on the metallic transition at high temperature and high pressure. Therefore, basing on density functional theory, We do a systematic study on the state equation, phase transformation, decomposition of molecules and metallic transition of silane at high temperature and high pressure, including solid silane’s thermodynamic stable structures at 25-300 GPa, the decomposition of liquid silane at 0-200 GPa, metallic transition, band gap and dc conductivity at 1000 K and 4000 K.Our research results can be summarized in the following two aspects:1. High-pressure phases of solid silane are predicted by using particle swarm optimization in pressure region from 25 to 300 GPa. The molecular crystal structure prediction method was used to search the stable structures of silane. At 55 GPa, three stable phases were found, two of them are in I-42 d and I41/a structures and in a good agreement with the experimental results of diamond anvil cell(DAC). A new phase is predicted, which has C2/c structure. Increasing with pressure, the I-42 d phase becomes unstable, only C2/c phase is coexistence with I41/a between 55 and 240 GPa, and both of them keep in insulated state. At pressure increased to 240 GPa, crystal silane transforms to Pbcn structure and becomes a metal. This work well explains the experimental equation of state at high pressure, and eliminates the possibility that loss of transparency for silane at 60 GPa is due to the metallization.2. Using the molecular dynamics simulations based on density functional theory, we calculate the equation of state for fluid silane; combining the pair correlation functions(PCF), we analyze silane molecule decomposition and it’s atomic reorganization at 1000 K 、 4000 K and 0-200 GPa. We identify the chemical decomposition of fluid silane is the first-order phase transition, which is correspond to the pressure’s discontinuity on the isotherm at 1000 K. An insulator-to-metal transition has been observed with the decomposition of fluid silane by the calculation analysis of band gap and dc conductivity...