First Principles Study Of A₂B^?B^?X₆ And A₂B^?X₆ Halide Double Perovskites

A₂B^?B^?X₆and A₂B^?X₆ halide double perovskites have good mechanical stabilities,thermodynamic stabilities,optical absorption coefficients and photoconductivities,which can be used for the X-ray photoelectric conductors or absorption layers in solar cells.Experimental researches on structures and physical properties of Cs₂Au₂X₆(X=Cl,Br,I),K₂TeX₆(X=Cl,Br,I)and Rb₂BX₆(B=Ti,Se,Pd;X=F,Cl,Br,I)are relatively plentiful,but the theoretical studies of the above-mentioned halide double perovskites have not been carried out yet.Therefore,this work analyzes effects of space groups,halogen atoms or B-site atoms on the structures and physical properties of Cs₂Au₂X₆(X=Cl,Br,I),K₂TeX₆(X=Cl,Br,I)and Rb₂BX₆(B=Ti,Se,Pd;X=F,Cl,Br,I)double perovskites using the principles calculations based on density functional theory.Cs₂Au₂X₆(X=F,Cl,Br,I)double perovskites are tetragonal crystals,and their bond lengths of Au-X and Cs-X increase with the atomic number of X increasing.Au_(2a)and X_(4e)as well as Au_(2b)and X_(8h)atoms form ionic bonds with small covalent components,while Cs_(4d)and X_(4e)as well as Cs_(4d)and X_(8h)atoms completely form ionic bonds.All Cs₂Au₂X₆(X=F,Cl,Br,I)double perovskites have very obvious elastic anisotropy.Expect for Cs₂Au₂F₆ double perovskite which lies in the critical point between brittleness and ductility,Cs₂Au₂X₆(X=Cl,Br,I)double perovskites are ductile.Cs₂Au₂X₆(X=F,Cl,Br,I)double perovskites are all direct band gap semiconductors with the theoretical band gaps of 2.410,2.008,1.609,and 1.371 e V for X=F,Cl,Br and I,which are very close to the experimental ones.Their top valance bands are mainly contributed X-p and Au-5d states,and their bottom conduction bands are predominately determined by X_(8h)-p and Au-5d states,while the conduction bands away from the Fermi surface mainly consist of X-s,Au-6s/6p and Cs-6s/6p states.The effects of all atomic sites,especially the,Au X₂-^-and,Au X₄-^-clusters with different valences of Au atoms,in the crystal structures of Cs₂Au₂X₆ on their electronic and optical properties have been clarified.As the increasing atomic number of X-site,optical peaks of Cs₂Au₂X₆(X=F,Cl,Br,I)move toward low energy region,and their amplitudes have also increased.By studying the effects of the space groups or the atomic number of X-site on the physical properties of K₂TeX₆(X=Cl,Br,I)double perovskites,the results reveal that in their crystal structures,X and K atoms form X-K ionic bond,while X and Te atoms form X-Te ionic bond with covalent components.For K₂TeBr₆double perovskites with three different space groups,the Br-Te bond lengths are very close.However,for the monoclinic K₂TeX₆(X=Cl,Br,I)double perovskites,as the atomic radius of X increases,the X-Te bond lengths increase with the decrease in covalent bond components.All the K₂TeX₆(X=Cl,Br,I)double perovskites exhibit obvious elastic anisotropy.In the electronic structures of K₂TeX₆ double perovskites,the top valence bands are primarily contributed by X-p(Cl-3p,Br-4p,or I-5p)and Te-5s orbits,and their bottom conduction bands are dominated by Te-6p orbit.The effective masses and carrier mobilities exhibit distinct anisotropy due to the anisotropic crystal structures.As the atomic number of X increases,the effective masses of electrons and holes gradually decrease,while the carrier mobilities increase.Especially,the higher carrier mobilities of monoclinic K₂Te₂I₆perovskite derivative are caused by its smaller effective masses.This work investigates the structues and physical properties of B-and X-dependent Rb₂BCl₆(B=Ti,Se,Pd)and Rb₂PdX₆(X=F,Cl,Br,I)double perovskites.Rb₂BX₆(B=Ti,Se,Pd;X=F,Cl,Br,I)are indirect band gap semiconductors,and their top valence bands are dominated by X-p(F-2p,Cl-3p,Br-4p,or I-5p)states,and their bottom conduction bands are decided by Ti-4d/Se-5p/Pd-5d states.With the increasing atomic number of B-or X-sites,the band gaps of Rb₂BCl₆ and Rb₂PdX₆gradually decrease,and their optical characteristic peaks move to the low energy region.The F-and Cl-based ones had high imaginary dielectric functions,high extinction coefficients,high absorption coefficients,and low reflection coefficients in the ultraviolet light range.As for the Br-and I-based double perovskites had wide optical absorption properties in the visible light range.The lattice vibrations of Rb₂BX₆ show that the high-frequency vibration modes(T_1u,T_2g,E_g,and A_1g)are derived from BX₆^2-clusters while the low-frequency vibration modes(T_2g and T_1u)are contributed by Rb atoms.