First Principles Study On Thermoelectric,optical And Topological Properties Of Ternary Compounds

In the past decades,information and energy materials have become a hot topic in the field of condenser matter.Information technology is evolving with the rapid development of spin electronics,and it also encourages people to search new spin-electronic materials to meet new technical requirements.Energy issues have always been the focus of attention of all mankind,especially in recent years with the depletion of traditional petrochemical energy and environmental pollution being more and more seriously.This requires people to develop new clean,renewable,low-cost energy materials.Some ternary compounds,such as Half-Heusler compounds ABX and perovskite structural compounds ABX₃,will have potential application because of their special band structure,topology and photoelectric properties in the field of information technology and clean energy.Ternary Half-Heusler compounds not only can be used as a semimetal or topological insulator,but also were widely studied as thermoelectric material by theory and experiment methods.The perovskite type ABX₃ is expected to be an important solar energy absorption material because of its clean and pollution-free,renewable,convenient advantages.The main contents of this thesis include the following aspects:（1）Based on first-principles calculations,we studied the lattice constant and atoms substitution tunable topological phase transition in the half-Heusler compounds HfIrX（X = As,Sb,Bi）.At the equilibrium cubic crystal structure and excluding SOC,HfIrAs and HfIrBi are topological nontrivial semimetals,while HfIrSb is a trivial topological insulator.This is because that the “internal pressure” lifts the s-type Г₁ band above p-type Г₅ bands in HfIrSb.When SOC is included,HfIrAs and HfIr Sb become topological insulator,and normal band insulator,respectively,while HfIrBi is still a topological semimetal.When we induce compressive stress in theab-plane of HfIrBi,it becomes a Weyl semimetal,with eight Weyl-Points（WPS）at（±K_x,0,±K_z）,（0,±K_y,±K_z）,K_x = K_y = 0.023 ?^-1,K_z = 0.108 ?^-1.（2）Strain dependence of the electronic structures,thermoelectric and topological properties of half-Heusler compounds ZrIrX（X=As,Sb,Bi）are investigated by using a modified Becke and Johnson exchange potential.At the equilibrium lattice constants,all the three compounds are trivial insulators and good thermoelectric materials with their Seebeck coefficient S and the power factor over relaxation time S2σ/τ as large as 1180（μV/K）and 41(10¹⁰W m^-1K-2s^-1),respectively.The compressive strain（smaller lattice constant）enhances the band gap,while the tensile strain（larger lattice constant）decreases the band gap,mainly because of the pressure sensitive s-typed conduct band at Γ-piont.Their absorption efficiencies increase from compressive strain to tensile strain.The Seebeck coefficients and the power factors of p-type doped ZrIrAs and ZrIrBi increase from tensile strain to compressive strain,and the critical value of direct-indirect gap transition can realize the bigger Seebeck coefficient and power factor for n-type,which can be understood by the effiective mass of the density of states（DOS）.At some specific tensile strains the compounds become Dirac-semimetals,with the s-typed band Г₆ below p-typed band Г₈,in the cubic phase.When we compress the a（b）-axis and elongate the c-axis of the compounds,they become the typical Weyl semimetals,with eight Weyl-Points（WPS）at（± K_x,0,± K_z）,（0,± K_y,± K_z）,K_x = K_y =0.008?^-1,K_z =0.043?^-1.（3）Based on the first-principles and Boltzmanns transport theory,the electronic structures,optical and thermoelectric properties of Ternary half-Heusler compounds TaIrGe were investigated.Spin orbit coupling（SOC）removed the degeneracy of VBM,and then obvious decreased the Seebeck coefficients and power factor.The band structures have been greatly modified by the strain.From the compressive strain to tensile strain,the band gaps increase gradually,and the absorption coefficient of the TaIrGe has an obvious red-shift.The effective mass of VBM and CBM gradually increase with lattice constant increasing from-4% to 4%.The thermoelectric properties has the significant strain dependence,the Seebeck coefficient and power factor both gradually increase with the strain from-4% to 4%.thermoelectric properties improved obviously by tensile strain.（4）The electronic structure and optical properties of the new solar cells absorber material: mixed perovskites CsSn_xPb_1-xI₃ are studied with mBJ+SOC（modified Beak Johnson approximation plus spin-orbit coupling）method.The band gap of the serial of compounds almost quasi-linearly reduces with increasing Sn content from 0.96 eV（x = 0）to 0.16 eV（x = 1）.Optical absorption coefficient revealed a progressive red shift with the increment of the Sn content,accompanying with the absorption edge broadening.The absorption coefficient and Ideal Power Absorption Coefficient（IPAC）increase greatly with the Pb atoms being partially substituted by Sn atoms.The pure CsSnI₃ has the highest IPAC,but it is unstable in the air because the Sn²⁺ will be oxidized to Sn⁴⁺.So our results indicate that partially substituted CsSn_xPb_1-xI₃ might be the good solar cell absorption material.（5）The crystal structures,electronic structures and optical properties of mixed halides perovskites CsAX₂X’（A=Ge,Sn,Pb;X,X’=Cl,Br,I）compounds are studied by the first-principle calculations.Band structure calculations,with mBJ+SOC（modified Beak Johnson approximation plus spin-orbit coupling）method,indicate that these compounds are semiconductors with the direct band gap ranging from 0.32 eV to 1.97 eV.By mixing the halogen elements（Cl,Br,I）and changing the pnictide elements（Ge,Sn,Pb）,we find the way to tune the band gap of the solar energy absorption materials.We propose several candidates for environmentally friendly solar energy absorbing materials with the absorption power as high as 1052 W/m2.