Theoretical Investigation On The Behavior Of Typical Metal Hydrides And Other Related Systems Under High Pressure

Under ultrahigh pressures, crystal or electronic structure for crystalline solid may be changed, which induce the changes of the material mechanics, electricity, magnetism, superconductivity, and other macroscopical properties. The process and mechanism of phase transition is a very rich exploration area in the investigation of high-pressure physics, which provide an important headspring for the innovation and development of material science.In this work, we carry out density functional calculations on the behavior of typical metal hydrides and the related systems under high pressure, using the pseudopotential and plane wave method as implemented in the CASTEP code. We have explained the reason for the different forms of pressure-induced electron transfer reasonably, using Mulliken Population Analysis. The investigations include the pressure-induced magnetic phase transition in nickel hydrides; the behavior of inert gas helium in metal nickel and the discussion on the magnetic properties of system under high pressure; the physical mechanism of the pressure-induced metallization in light metal hydrides; the process of molecular dissociation and mechanism of the pressure-induced structural phase transitions in solid diatomic molecule I₂. The main contents are as follows:(1) The pressure-induced magnetic phase transition in nickel hydrides. In the past decades, metal-hydrogen systems have attracted much attention in both theoretical and experimental investigations, due to the technological applications, such as a hydrogen storage material, as a catalyst in chemical production and experiments. Besides that, the investigation of nickel-hydrogen systems provides a proper object on the proton-metal interactions in materials for fusion reactors. The magnetic property of metal Ni is changed due to the addition of hydrogen. But the magnetic transition induced by the integrative effect of pressure and hydrogen is lack. In this part the effects of pressure on NiHx with different hydrogen concentrations (x=0, 0.25, 0.375, 0.5, 0.625, 0.6875, 0.75, 1) have been extensively studied. Finding: a) At normal pressure, with the increase of H concentration, the Ni-H systems indeed exit a ferromagnetic (FM) to paramagnetic (PM) phase transition, with the critical value of x≈0.7, which is in reasonable agreement with the experimental results of x=0.65. b) A pressure-induced FM to PM phase transition has been predicted. The transition pressure from FM to PM decreases with increasing hydrogen concentration. The phase diagram of hydrogen concentration vs. pressure up to several megabar has been obtained. c) The mechanism of the magnetic transition induced by H concentration and pressure is revealed. The hydrogen-doping induced magnetic transition is caused by the electron transfers from 4p and 4s to 3d inside the Ni atom and from 4p and 4s of Ni atoms to H 1s, while the pressure-induced magnetic transition is mainly attributed to the electron transfers from the nearest-neighbor Ni 4p to H 1s, from the nearest-neighbor Ni 4p to the further Ni 4s, and from 4p to 4s and 3d inside each Ni atom.(2) Under high pressure, the formation of NiHe_0.25 compound and the discussion on the magnetic properties of the system. Understanding of light elements behavior in metal is a main issue on the investigation and development of fusion reactor materials. At normal pressure, inert-gas atoms are essentially insoluble in most solids. This leads to gas-atom precipitation, bubble formation, and eventually to a complete destruction of the material. But the pressure effect on the formation of helium bubble and on the magnetic properties in Ni-He system is lack. In this part, the spin-polarized electronic properties calculation on Ni-He system with the He concentration 0.25 is investigated. Analysis the stability of various structures from changed energy, obtain the diffusing potential well among interstitial sites, and forecast the most possible diffusing path of helium atom in metal Ni and the most stable sites of helium atom in metal Ni under high pressure. The interaction between helium and Ni is described by analyzing the atomic orbital population, the bonding population, the bonding structure and the density of states. Finding: a) In Ni-He system with the He concentration 0.25, when the pressure is high enough, helium-bubble cann't create and the compound of NiHe_0.25 can exist. b) Under pressures, the effect of He on the magnetic properties of Ni is stronger than that of H. c) In NiHe_0.25, He is as a cation, but the charge of He is almost stable with increasing pressure. d) Under pressures, the chemical activity of He is enhanced and even stronger than that of H. All the above is unimaginable at normal pressure.(3) The pressure-induced metallization in light metal hydrides. Light metal hydrides, as a potential high-Tc conventional superconductor and potential materials with enhanced storage capacity as solid state sources for hydrogen cells, have been attracted much attention by people in theoretical and experimental investigation. According to the BCS theory, before the system is as a superconductor, it should be with metallicity. But at present, the suggestion that the light metal hydrides is as a potential high-Tc superconductor is confirmed insufficiency. So, in a series of light metal hydrides LiXH₃ and XLiH₃ (X=Be, B or C) with perovskite lattice structures, we investigate the cause of formation of the metallicity in them and the physical mechanism of the pressure-induced metallization in LiBeH₃. Finding: a) Large energy gap of LiBeH₃ indicates that it is an insulator, but other investigated hydrides are metallic. b) The pressure-induced metallization of LiBeH₃ is found at about 120 GPa, which is attributed to the increase of Be-p electrons with increasing pressure. c) The electronegativity of the p electrons of X atom is responsible for the metallicity of the investigated LiXH₃ hydrides, and X-p electrons play an important role on the Fermi level of LiXH₃ hydrides. d) The electronegativity of the s electrons of X atom plays an important role in the metallicity of the investigated XLiH₃ hydrides. Furthermore, in the series of XLiH₃ hydrides, there is an electron transfer from X-p to X-s due to the formation of hydrides, which lowers the host Fermi level and makes the hydrogen-related levels lie close to it.(4) Investigating the process of pressure-induced molecular dissociation and mechanism of the pressure-induced structural phase transitions in solid I2. In diatomic molecular solids, the successive pressure-induced transitions, including metallization, molecular dissociation and superconductivity, make this research field more attracting and challenging. Furthermore, the investigations on dense diatomic molecular solids such as oxygen, nitrogen, and halogen can give important and valuable information for understanding the dense metallic hydrogen. By investigating the diatomic molecule I₂ under high pressure, finding: a) The dependence of lattice parameters on pressure indicates that the first structural phase transition from phase I to phase V occurs at about 20 GPa, which is agreement with the results from equation of states and elastic constants. b) From the pressure dependence of our elastic constants for solid iodine in phase I, it is found that the structural transition from phase I to phase V is due to the softening of elastic constant C₄₄. c) The incommensurate phase V is mimicked by a periodic crystal structure and in our calculation, the modulated phase V is thermodynamically and mechanically stable from 20 to 29 GPa. The electronic and optical properties indicate that the modulated phase V is not a monatomic phase but an intermediate state between a molecular and a monatomic state. This phase has more molecular characters under lower pressures, but it has more monatomic characters under higher pressures.