First-principles Study Of The High Pressure Mechanical,Electronic And Optical Properties Of Alkali Metal Hydrides And Alkali-earth Metal Hydrides

New energy materials and optical materials will undergo the microstructural deformation and phase transition if they encounter the influence of external extreme environment(e.g.high temperature,high pressure,strong laser irradiation,etc.).The change of microstructure will affect the macroscopic properties of materials,such as anti-damage properties,photoelectric properties,electromagnetic properties,safety performance,etc,and these properties are closely related to people's production and livelihood.The microstructural changes of materials are not conducive to the understanding and control of these properties,also affect the scientific research and practical application.Therefore,theoretical and systematical study on the microstructural deformation,electronic properties,charge distributions in crystals and the excitation mechanism of optical properties are necessary,and have some important guiding significance to the experimental researches and applications for these materials.In the dissertation,based on the density functional theory,some important conclusions are obtained as follows:From the usual condition of equal enthalpy,high pressure phase transition behavior of LiH,NaH,KH,RbH and CsH was investigated.The phase transitions of LiH,NaH,KH,RbH and CsH from B1 to B2 phase occur at 336.2,30.5,4.1,3.1 and 2.3GPa,respectively,the phase transition pressures decrease with increasing atomic number.The calculated results by the stress-strain relation indicate that,at ground states,all the B2 LiH,NaH,KH,RbH and CsH are elastic instable.At high pressure,stretched or compressed elastic constant C11,shear constant c' and bulk modulus B are the major factors,which cause to the high pressure phase transition for cubic alkali hydrides.B1 and B2 LiH,NaH,KH,RbH and CsH have the trend of plastic transformation when pressures exist.The calculated results of band strucrures with the HSE hybrid functions indicate that all the B1 alkali hydrides are direct band gap materials,and the band gaps increase firstly and then decrease with the increasing atomic number;All the B2 alkali hydrides are indirect band gap materials,and the band gaps increase with increasing atomic number.The band gaps of B1 and B2 LiH decrease with the pressure increasing,and there is a trend of phase transition that from semiconductor to conductor for LiH.Both valence bands(VBs)and conduction bands(CBs)of B1 and B2 LiH are non-local.The VBs of B1 and B2 NaH,KH,RbH and CsH have strongly locality,but CBs are non-local.At the same time,there also have some influence on the band structures of alkali hydrides when pressures exist.The results of Mulliken charge population indicate that,at ground state and high pressure,the charges mainly distribute around the alkali metal and H atoms,and the charge popuations mainly arise from the alkali metal atoms p and H 1s states,and the charges transfer from alkali metal atoms to H.In LiH,NaH,KH,RbH and CsH crystals,some influence about the charge distributions were found when pressures exist.The results from the complex dielectric function and Kramer-Kronig relation indicate that,at ground states and high pressure,all the alkali hydrides are suitable as optical dielectric materials,and exhibit a metallic behavior when their real dielectric function?1(?)<0;All the alkali hydrides have a wide absorption region,and the main absorption parts locate in the ultraviolet(UV)region,suitable as UV absorption materials;The extinction coefficients are similar with their imaginary part of dielectric and can be explained by photoelectron transition mechanism of the imaginary part of dielectric;Except for B2 RbH,the reflectivity and energy loss function of cubic LiH,NaH,KH,RbH and CsH are wider,and unsuitable for optical transmission materials because reflection and energy loss are larger when photons pass through them,but suitable as antireflection materials at high frequency region.The optical properties are affected by the pressure,and their curves have different degrees of red shift or blue shift.The results of phonon spectra show that ?-BeH2,?,?-MgH2,P63/MMC-CaH2(SrH2,BaH2)and P6/MMM-SrH2 are dynamically unstable at ground state.The results of the calculated band structures with GGA and HSE methods show that the calculated band gaps with the HSE are larger than GGA.?,?-BeH2 and ?,?-MgH2 are direct band gap materials;?,?,?-BeH2,?,?,?-MgH2 and PNMA,P63/MMC-CaH2(SrH2,BaH2)are indirect band gap materials;P6/MMM-CaH2(SrH2)are conductors.At the same time,the common characteristics of the band structures for these alkaline earth metal hydrides were analyzed in detail.(1)Except for BeH2,the VBs of MgH2,CaH2,SrH2 and BaH2 are local;(2)The CBs are non-local;(3)The VBs and CBs of P6/MMM-CaH2(SrH2)cross and display a metal behavior.The results of Mulliken population indicate the charge populations of the alkaline-earth metal hydrides mainly origin from H 1s and alkaline-earth metal atoms s and p states.The charges transfer from alkaline-earth metal atoms to H atoms.All the BeH2,MgH2,CaH2,SrH2 and BaH2 are mixture bonding materials,and the covalent character is obvious.The dielectric peaks of alkaline-earth metal hydrides mainly arise the transition from H 1s and alkaline-earth metal atoms p states in VBs to alkaline-earth metal atoms s,p and d states in CBs,or transitions themself.All the alkaline-earth metal hydrides show a metal behavior when ?1(?)<0;They have a wide absorption region,and the main absorption parts locate in the UV region,and they are all good UV absorbing materials;The extinction coefficients are similar with their imaginary part of dielectric and can be explained by photoelectron transition mechanism of the imaginary part of dielectric;All the maximum energy loss are located in the UV region,and energy loss spectrum of ?,?,?-BeH2 and a-MgH2 are broad,therefore,they can't be used as optical transmission materials.The energy loss spectra of other alkaline earth metal hydrides are sharp,and they are suitable as optical transmission materials,and also suitable as antireflection materials at high frequency.Comparing with the GGA results,the curves ?(?),?(?),n(?),k(?),R(?)and L(?)with the HSE method for BeH2,MgH2,CaH2,SrH2 and BaH2 blue-shift toward to high energy regions,and intensities have a relative change.