Mechanism Of Covalent Bond Induced SHG Response In Lead Borates

Non-linear optical materials are important photoelectric information functional materials,and the development of laser technology is closely related to the development of non-linear optical materials.At present,nonlinear optical materials with moderate birefringence and large frequency doubling have attracted widespread attention,and people have put forward higher requirements for the comprehensive performance of crystal in practical applications.Therefore,design and synthesis of new nonlinear optical materials with excellent optical performance is very important.In this work,first-principles methods are used to study linear and nonlinear optical properties of Pb₂BO₃X and Pb₂B₅O₉X(X=Cl,Br and I).The calculated values of band gap,refractive index,birefringence and SHG coefficient are in agreement with experimental values.By the analysis of electronic structure,orbitals,SHG density and Born effective charges of these materials,SHG response and birefringence are thoroughly investigated.The research results show that Pb atom in lead borate is a lone pair of electron with stereochemical activity.The halogen and covalent network of Pb₂BO₃groups are the reasons for SHG coefficient of lead borate Pb₂BO₃X(X=Cl,Br and I),and the covalent network also has an obvious contribution to birefringence.The contribution of Pb₂B₅O₉X(X=Cl,Br and I)mainly comes from the Pb sp state,the O p orbital and the halogen p orbital,and halogen has a great contribution to birefringence.Post-transition metal cations are widely used in rechargeable batteries,solar cells and birefringent compounds.Our research group used first-principle methods to calculate electronic structure and optical properties of a series of post-transition metal halide materials(ternary Cs Sn X₃(X=Cl,I),Cs Pb X₃(X=Cl,Br and I),and binary MX₂(M=Sn,Pb;X=Cl,Br)).By research methods such as crystal orbital analysis and atomic orbital energy level difference analysis,it is found that the degree of stereochemical activity of the material is Cl>Br>I.Further analyzed orbital energy levels and projected density of states of Cs Sn X₃(X=Cl,I),Cs Pb X₃(X=Cl,Br,and I)and MX₂(M=Sn,Pb;X=Cl,Br)material.In halides,the degree of stereochemical activity of lone pair electrons is related to the energy level difference between M s and X p(M is a metal cation,X is a halogen).The smaller the energy level difference,the stronger the stereochemical activity of cations are.The refractive index and birefringence of the material are greatly affected by M-p and X-p orbitals.The higher the ratio of the integral area of cationic-p orbital near the Fermi surface,the greater the birefringence value is.