First-principles Studies On Equation Of State And Metallization Transition For Hydrogen (Deuterium) At High Pressure And High Temperature

Hydrogen is the simplest element in the universe, which is one of the classical systems of condensed matter problem. In the modern natural science and engineering technology, many problems such as metallic hydrogen, high temperature superconducting, inertial confinement fusion, interstellar medium and so on, are all involving changes of state properties of hydrogen and its isotope under high temperature and high pressure. It is of important scientific significance to explore its structural changes (lattice structure, electronic structure and the phase behavior, including solid, liquid, vapor and plasma phase etc.) and the influence on the interaction between macroscopic physical properties (including thermal, electrical, optical properties, etc.) of composition of the particles (molecules, atoms, ions, electrons), and thus it is important to understand the condensed matter physics properties of hydrogen under high temperature and high pressure.The material, on the macro, can be characterized by the decrease of the volume and the increase of internal energy under high temperature and high pressure, and the space of atoms and molecules would be shorted by the volume compression. It is thought that the spatial distribution of the electron density will change, with the atomic distance shortage, the local electronics have been transformed to delocalization, and thus leading to the changes of the physical and chemical properties of material in quantum mechanics. It has been found that there are average 3-5 transitions in the process of the pressure increase from normal atmospheric pressure to 100 GPa (～1MPa). Although the electronic structure of hydrogen is the simplest, the phase diagram is incredibly complicated.In recent years, there are still quite controversies on the change relationships of the pressure, temperature, volume, the conditions of the metallization transition and the thermodynamics areas, especially, there are lacking of relevant experimental data. Therefore, this article introduces the research background and research methods, the equations of state for solid hydrogen and fluid hydrogen are described based on density functional theory, and the changeable regulations that the density of solid and liquid hydrogen, liquid hydrogen dc-conductivity changing with the temperature and pressure are researched.There are two aspects about our research results as follows:1. By using the density functional theory (DFT) with the generalized gradient approximation (GGA), the equation of state (EOS) and phonon spectra of solid hydrogen Pca21 structure are calculated. At 310 GPa, our simulations predicte a transformation from Pca21 phase into the ordered molecular metallic Cmca phase (4 molecules/cell). Within this phase, the band structure crosses the Fermi surface and provides theoretical support for metallization of solid hydrogen.2. The molecular-dynamics simulations based on density functional theory have been explored to obtain the pressure of shock compressed fluid deuterium and study the properties of warm dense hydrogen molecular dissociation, electronic structure characteristics in density range of 0.30-1.10 g/cm3 and temperature from 1000 K to 3000 K. We find clear evidence of pressure discontinuity at the isotherm of 3000 K by large-scale simulations, which also confirm the first-order nature of this transition. The clear evidence indicates that the molecular-phase to atomic-phase is a first-order transition, and accompanying sharp increase in the electrical conductivity also shows this transition corresponds to the nonmetal-to-metal change. From the electrical conductivity, we predict the coexistence region over a wide range at different densities or temperatures. A new transition line has been located in the phase diagram.In this thesis, the influences of temperature and pressure on the physical properties of hydrogen were analyzed based on the density functional theory (DFT). The existence of metallic hydrogen was confirmed on the condition of solid state. And it is found that the number of particles of the primary principle of molecular dynamics simulation has effects on the judgments of fluid hydrogen first-order phase transition, there was existence of first-order phase transition characteristics for the fluid hydrogen at 3000 K when the larger simulation sizes were used during the simulation process for the first time. And thus, a complete equation of hydrogen state data was acquired preliminary. The existing electrical conductivity and pressure of shock compressed results have been explained reasonably.