Ab Initio Studies Of Solid Halogens And Halide Under High Pressure

The effect of high pressure on molecular systems has been a central issue of fundamental physics and chemistry as well as planetary sciences. Cohesion of simple molecular solids occurs through forces of very different strengths: covalent, ionic, van der Waals, and hydrogen bonds. Pressure drives materials to states of higher density and gives rise to competition among those chemical bonds, structural instabilities, and changes in electronic properties. A simple picture suggests that all molecular systems must collapse on compression to form closed-packed structures and go over into metallic states at sufficient high pressures. However, the diversity of a process toward their destruction in real substances has manifested itself in numerous experimental observations. For instance, diatomic molecules crystals H₂, N₂, O₂, and I₂ are widely known to exhibit unexpected phases and complex phase diagram. Another class is hydrogen-containing molecules, such as H₂O, NH₃, and HCl, they turn out to be hydrogen bond symmetrization under high pressure. Metal tetraiodides MI₄ (M=Ge, Sn) are examples of another class. They undergo pressure-induced amorphization and become metallic glasses, which are quite common in materials having tetrahedral coordination. Among various factors affecting on the response of molecular crystals to compression, the shape of a molecule and intermolecular interactions may be of particular importance, because the anisotropy of chemical bonds, crystal structure, and electronic properties strongly depend on the shape of molecules composing a crystal and intermolecular interactions between the individual building blocks. Most recently, with the help of improved theory and computational capability, ab initio calculation based on the density functional theory has been used widely in the condensed matter physics, quantum chemistry, and material science. And it has been a common research tools except for theoretical and experimental method. Metal hydrogen is the most important one of top ten physical problems in the 21st century. Most likely it is a room temperature superconductors and good energy materials. Pressure is the most effective method to obtain metal hydrogen. Because the hydrogen molecule internally bonding is very strong, so far, there is no metal hydrogen observed in laboratory. As Ashcroft pointed out, hydrogen is regarded naturally as the first element of the halogen group. Therefor, we detailedly study the pressure induced molecular dissociation, metallization and superconductivity of solid halogens and related halide, which are also valuable for providing insight into the metallic hydrogen.(1) Effect of nonhydrostatic pressure on superconductivity of monatomic iodine. The superconductivity of iodine had been successfully discovered with T_c = 1.2 K at 28 GPa. It was reported that the T_c of monatomic iodine decreased with pressure at first but started to increase with pressure for the highest-symmetry phase IV, the face-centered cubic phase. The mechanism of such a superconductivity with pressure is still unclear. So, we have presented an ab initio investigation of the hydrostatic and nonhydrostatic pressure effects on the superconductivity of monatomic iodine.It is shown that the T_c of both phase II and phase III under hydrostatic pressures are in agreement with the experimental data, while the T_c of phase IV under hydrostatic pressures decreases with increasing pressure, contrary to the experimental results. In order to explore the origin of difference between experimental and theoretical results, we have studied the effect of non-hydrostatic pressure on the superconductivity of monatomic iodine, and found that the symmetry of phase IV changes from face-centered cubic to face-centered orthorhombic (fco) under anisotropic stresses. Further calculations show that the T_c of this fco structure increases with increasing pressure, in good agreement with the experimental results, which is mainly attributed to the non-hydrostatic pressure-induced enhancement of the electronic density of states at the Fermi level and electron-phonon coupling matrix element 2>.(2) Crystal structure and superconducting properties of monatomic bromine under high pressure. The monatomic phase transition sequence of iodine had been observed by X-ray diffraction experiment. A similar scheme of phase transformations can be expected for bromine, but experimental results are much scarcer than those in the iodine case which restricts our understanding of the nature of bromine under high pressure. So, the crystal structure and superconducting properties of monatomic bromine under high pressure have been studied by ab initio calculations.We have found the following phase transition sequence with increasing pressure: from body-centered orthorhombic (bco, phase II) to body-centered tetragonal structure (bct, phase III) at 126 GPa, then to face-centered cubic structure (fcc, phase IV) at 157 GPa, which is stable at least up to 300 GPa. The calculated superconducting critical temperature T_c = 1.46 K at 100 GPa is consistent with the experimental value of 1.5 K. In addition, our results of T_c decreases with increasing pressure in all the monatomic phases of bromine, similar to monatomic iodine. Further calculations show that the decrease ofλwith pressure in the phase IV is mainly attributed to the weakening of the"soft"vibrational mode caused by pressure.(3) Hydrogen bond symmetrization and superconducting phase of HBr and HCl under high pressure. Hydrogen bonds are quite pervasive in a broad range of fields including physics, chemistry, biology, and materials sciences. Besides, hydrogen bond symmetrization is an important high pressure phenomenon. The pressure-induced hydrogen bond symmetrization in hydrogen halides (HBr, HCl, and DCl) have also been observed by Raman and infrared measurements. In addition, hydrogen in HBr and HCl can lead to other interesting properties under high pressure. Recently, theoretical or experimental studies have reported that these hydrogen compounds such as SiH₄, GeH₄, SnH₄, YH₃, ScH₃ and LaH₃ present a high superconducting critical temperature. HBr and HCl are simple diatomic molecules forming hydrogen bond in condensed state. Therefore, the studies on HBr and HCl can provide theoretical guidance to other hydrogen bonding system.Ab initio calculations are performed to probe the hydrogen bonding, structural and superconducting behaviors of HBr and HCl under high pressure. The calculated results show that the hydrogen bond symmetrization (Cmc2₁→Cmcm transition) of HBr and HCl occurs at 25 and 40 GPa, respectively, which can be attributed to the symmetry stretching A₁ mode softening. After hydrogen bond symmetrization, a pressure-induced soft transverse acoustic (TA) phonon mode of Cmcm phase is identified, and a unique metallic phase with monoclinic structure of P2₁/m (4 molecules/cell) for both compounds is revealed by ab initio phonon calculations. This phase preserves the symmetric hydrogen bond and is stable in the pressure range from 134 GPa to 196 GPa for HBr and above 233 GPa for HCl, while HBr is predicted to decompose into Br₂+H₂ above 196 GPa. Perturbative linear-response calculations predict that the phase P2₁/m is a superconductor with T_c of 27³4 K for HBr at 160 GPa and 9¹4 K for HCl at 280 GPa.(4) Pressure-induced amorphization and recrystal of tin tetraiodide molecular crystal. In recent years, pressure-induced amorphization (PIA) has attracted extensive experimental and theoretical interest such as H₂O, SiO₂, P, etc. Tin tetraiodide SnI₄ molecular crystal is also observed to show amorphous under pressure. Although there have been a variety of experimental studies on SnI₄ under pressure, the structure of PIA forms is still controversial. In addition, the amorphous recrystallizes to a nonmolecular crystalline phase III (CP-III) at 61 GPa, but the crystal structure is not clear. Here we report an ab intio study that reveals the mechanisms controlling PIA in SnI₄, provides important insights pertaining PIA phenomena at large, and gives the structure of CP-III.Full geometry optimization show that, at 25 GPa, the lattice constants abrubtly split, volume significantly decreases, and distance of intramolecular Sn-I increase abruptly, while intermolecular and intramolecular I-I decreases suddenly. These indicate the tetrahedral molecular dissociate at 25 GPa, which is in good agreement with the experiment results. In addition, we obtained an amorphous structure through the classical molecular dynamics. The XRD and radial distribution function of this structure is consistent with the experimental resuts. At 60 GPa, the lattice constants changed abrubtly, indicating that the crystal phase (CP-III) occured. We firstly got a structure of CP-III with space group P2₁/c which has 40 atoms in unit cell. The XRD of our calculated structure is consistent with the experiment measure, indicating that our predicted structure is correct.(5) Pressure-induced molecular dissociation and superconductivity of boron triiodide. It is reported that BI₃ molecular structure transform to a monatomic phase at 6.2 GPa with the face-centered-cubic lattice of iodine atoms by X-ray diffraction experiment. Since the atomic X-ray scattering power of boron is only 5/53 of that of iodine or less, the boron atoms were not detected. So, the crystal structure of new phase is not clear. The monatomic phase becomes metallic at 23 GPa and exhibits superconductivity above 27 GPa by resistivity measurements.We got a new structure of BI₃ with space group P2₁/c which has 4 moleculars in unit cell. The XRD of P2₁/c is in good agreement with the experiment measure, indicating that our predicted structure is correct. The pahse I (P6₃/m) transforms to pahse II (P2₁/c) at 5.6 GPa, which is in well agreement with the experimental results. Moreover, the P2₁/c structure is dynamical and mechenical stability by phonon and elastic calculation. Another phase transition from insulator to metal phase occurs at 30 GPa which is primarily attributed to the band overlap. Perturbative linear-response calculations predict that the phase P2₁/c is a superconductor with T_c of 0.5 K.