Theoretical And Experimental Studies Of Halogen Compounds Containing Hydrogen Under High Pressure

High pressure can reduce the distance between atoms,overlap orbitals,break and recombine of chemical bonds,even distort the structure,and so on.Molecular systems are prone to structural phase transitions and even molecular dissociation due to their weak intermolecular interactions.Hydrogen,as the lightest element in the periodic table,according to BCS theory:hydrogen has a high Debye temperature and has been predicted to be a good candidate for the room temperature superconductor theoretically.And high pressure is the effective way to change a molecular crystal at normal pressure into a metallic phase.However,it has never been achieved experimentally,mainly because hydrogen has a small atomic radius and a strong bond between atoms,the metallization pressure of hydrogen is too high to be achieved with the current experimental technology.Encouragingly,halogen atoms,having very similar chemical properties with hydrogen,have attracted widespread attention.Chloride appears metallization at 100 GPa and dissociates into atomic phase at 130GPa,while bromine changes from insulator to metal at about 60 GPa,and then transforms into monatomic phase at 115 GPa.It was not until 2004 that Ashcroft mentioned the“chemical precompression”,people turned to hydrogen-containing compounds.Therefore,hydrogen halide comes into people's sight because of the abundant phase transition,and hydrogen chloride and hydrogen bromide both exhibit hydrogen bond symmetry and superconductivity under high pressure,which also gives a guide to other hydrogen-containing systems.Ammonium halide,as a member of the hydrogen-rich compound family,has attracted much attention due to the phase transition.In this paper,we selected three halogen compounds containing hydrogen to study their high-pressure behavior by means of Brillouin scattering experiment and first principles calculation,the achievements are as follows:1.We studied the elastic properties of hydrogen chloride systematically by synchrotron radiation X-ray diffraction and in-situ high pressure Brillouin scattering spectroscopy.We obtained the sound velocity variation along three isotherms,namely300 K,390 K and 470 K and stated the influence that pressure has on velocity in detail.We obtained the volume data from XRD experiment and used the third Birch-Murnaghan equation of state to fit the XRD data to obtain the density.The elastic constants,volume modulus,shear modulus and elastic anisotropy were obtained according to the density and the sound velocity.We also studied the influence of pressure and temperature on the elastic properties,finding that the elastic constants and modulus are monotonically increasing with pressure along each isotherm,and at low pressure,the increase of temperature will cause these values decrease slightly.We also determined the phase boundary between phase I and I'by calculating the mean of the least square error and the elastic anisotropy,which change significantly at the phase transition points of disordered to ordered phase.Finally,the phase diagram of hydrogen chloride was extended to high temperature-pressure range,which provides a powerful reference for other hydrides.2.We performed fixed-composition structure prediction of NH₄I with the cell size containing two,three and four formula units by the ab initio evolutionary algorithm up to 130 GPa.There is no doubt that we have found these three phases(?,?and?)that were confirmed experimentally,and the crystal structures and the pressures of phase transition agree well with experimental results.Still,two new phases with space group Ibam and Cm have been uncovered for the first time.The calculations of electronic band structure indicate the band gap of the two new phases decrease with increasing pressure.Then,considering the dissociation of NH₄Br and instability of HI,we compared the enthalpies of all predicted structures and NH₃,H₂,I₂,finding that phase?persists up to 74 GPa where NH₄I dissociates into NH₃,H₂and I₂,so the two new phases are metastable structures.And there is no imaginary frequency in phase V until 80 GPa,we conclude the dissociation is not caused by dynamic instability,maybe for the large radius and weak electronegativity of iodine atom and the weak intermolecular interactions.3.We searched the stable structures of ammonium chloride in the pressure range of 0 to 350 GPa,finding two new phases at high pressure,P2₁/m and Cmma,and confirmed the two phases are stable in the pressure range of 71-107 GPa and 107-350GPa according thermodynamic,mechanical and kinetic stability.Considering that high pressure may cause NH₄Cl to dissociate,we also have calculated the enthalpy differences of phase Cmma relative to NH₃(Pma2)and HCl(P-1)at 0 K.But the enthalpy of NH₃ and HCl is 4.2 e V higher than that of phase Cmma of NH₄Cl up to350 GPa.In addition,the crystal structures of ammonium chloride adopt the antiparallel orientation of ammonium ions in phase?,P2₁/m and Cmma.We also calculated the electron localization function and Bader charge of the P2₁/m phase and the Cmma phase,finding that only H atoms lose electrons,which are obtained by N and Cl atoms and the covalent bond between the hydrogen and the nitrogen goes all the way up to 350 GPa.We also explored the electronic band structures of the four stable structures and gave the plot of changes in band gap versus pressure,finding in the pressure range of phase?,the band gap increases with increasing pressure,the band gap values of phase?and two new phases decrease with pressure,metallization occurred at 350 GPa.The calculation of dieletric constant and optical absorption coefficient shows that pressure is an effective means of regulating the optical properties of materials.