The Investigation On Novel Structures And Properties Of LiB And LiBH Compound

Since the superconducting transition(T_c=39 K)of Mg B₂ was reported,the research of alkali and alkaline earth metal borides in the field of superconductivity has achieved fruitful results.Lithium boride and lithium borohydride compounds could show special bonding characteristics and some novel physical properties.In this paper,the structural characteristics and electrical properties of novel lithium boride and lithium borohydride phases are studied by using various theoretical calculation methods.The experimental synthesis of Li–B compounds is investigated by the method of high–pressure and high–temperature(HPHT)technology.The superconductivity of three new Li₂B structures are theoretically studied:hP6-,m S12-,and o C12-Li₂B.Electronic property calculations find that the three novel Li₂B phases are all metallic and the quasi-2D character in the band structure of hP6-Li₂B is similar to that of Mg B₂.Electron–phonon coupling calculations indicate that the proposed Li₂B structures have low superconductivity.Enthalpy difference analyses show that o C12-Li₂B may be formed by?-LiB and Li.The superconductivity of two metastable LiB₂ structures are investigated:P42/mmc-LiB₂ and I41/amd-LiB₂.The calculated Vickers hardness using Chen's formula indicates that the two metastable LiB₂ structures and previously proposed Cm-and P6/mmm-LiB₂ are potentially hard materials.Electron–phonon coupling calculations indicated that P42/mmc-,I41/amd-,and P6/mmm-LiB₂ structures have superconductivity and their T_c values are evaluated to be 20.1,12.6,and 31.4 K,respectively.The excellent mechanical and superconducting properties of LiB₂ compounds indicate this material has attractive prospects for industrial application and scientific research.The electronic structure and superconductivity of hexagonal Li₃B₂ and constructed hexagonal Li₂B₂H structures are systematically studied.A stable hexagonal Li₃B₂ phase is identified at 20 GPa by using CALYPSO structure search software.Replacing part of Li with H,a novel hexagonal Li₂B₂H structure is simulated.The superconducting temperature of Li₂B₂H structure is significantly higher than that of Li₃B₂ due to the introduction of the H layer.This provides a new idea for improving the superconductivity of layered alkali metal and alkaline earth metal borides.Five new LiBH structures are predicted:Imma-LiBH,R-3m-LiBH,P6₃mc-LiBH,Pmmn-LiBH,and C2/m-LiBH.Imma-LiBH and R-3m-LiBH are mechanically and dynamically stable at atmospheric pressure;P6₃mc-LiBH,Pmmn-LiBH,and C2/m-LiBH are stable under high pressure.Hydrogen adsorption energy calculations show that the physical adsorption occurs on the(001)surface of previously proposed Pnma-LiBH and(010)surface of Imma-LiBH at Li-top,B-top,H-top,B–B bridge,and Li-interspace positions.The adsorption on(001)surface of Pnma-LiBH at H-interspace site belongs to chemisorption.The(010)surface of Imma-LiBH has a negative hydrogen adsorption energy at H-interspace site,which is conducive to the release of hydrogen.Considering its conductivity,it can be applied as an electrode material for hydrogen fuel cells.The pressure–induced metallization and superconductivity in orthorhombic LiBH₂phase are studied.Theoretical calculation results indicate that the predicted LiBH₂structure turns its semiconductor into a conducting property in the pressure range of 150–200 GPa.The superconductivity of this structure appears at 300 GPa.Its?and T_c values increase with pressure due to the softening of transverse acoustic mode B_3u at the Y point.Moreover,the B_3u excessive softening at Y point makes LiBH₂ dynamically unstable above 700 GPa.The pressure–induced phonon softening may enhance the metallicity and superconductivity of the structure to a certain extent.Li–B compounds synthetic experiments are carried out by using HPHT method on hexahedral press.