Research On The Influence Of Cations With Lone Pair Electrons In Borates And Carbonates On Optical Properties

Nonlinear optical(NLO)crystals,that can extend the wavelength range of laser by frequency conversion technology,which have received extensive attention.Second harmonic generation(SHG)coefficient and birefringence are two important parameters of NLO crystals,which affects SHG conversion efficiency and phase matching ability,respectively.In the process of exploring NLO materials,introducing the cations with stereochemically active lone pair electrons(Sn²⁺,Pb²⁺,Sb³⁺,Bi³⁺),on the one hand,is favorable to obtain non-centrosymmetric structures,on the other hand,is beneficial to enhance the linear and nonlinear optical properties of compounds.However,it is found that the enhancement of lone pair electrons on SHG response is different.For example,the SHG responses of Bi B₂O₄F₂ and?-Bi B₃O₆ are 14.9 and 7.9 times that of KDP,respectively.Different lone pair electrons cations have different influences on the SHG response of compounds,what is the influence of the same lone pair electrons cation of polymorphism?For birefringence,the introduction of?-conjugated planar groups([BO₃]^3-,[CO₃]^2-,[NO₃]^-,[B₃O₆]^3-,[C₃N₃O₃]^3-)is usually one of the most effective ways to obtain large birefringence.In addition to introducing?-conjugated groups,how to design the new microscopic structure(such as introducing H atom)to further enhance the optical properties,under the new microstructure framework,the mechanism of the cations containing lone pair electrons on birefringence are also scientific questions worthy to study.In this work,the linear and nonlinear optical origin of targeted compounds are analyzed by first-principles calculations based on density functional theory,which is beneficial to better design linear and nonlinear optical materials.(1)To study two types of systems with lone pair electrons through first-principles calculations,one is polymorphism?,?,?,?,?-Bi B₃O₆ including same chemical composition.The other is Bi B₂O₄F and Bi B₂O₄(OH)including different anionic groups.It is found that stereochemical activity of the lone pair electrons is different in?,?,?,?,?-Bi B₃O₆.In Bi B₂O₄F and Bi B₂O₄(OH),the stereochemical activity of the lone pair electrons is induced not only by the interaction of Bi-O but also by the interaction of Bi-F.The synergistic effect of Bi-O polyhedral and anionic group determines the optical properties of the compounds.For birefringence,the parallel-aligned[BO₃]^3-groups are conducive to achieve large birefringence,both the[BO₃]^3-groups and Bi-O polyhedral with lone pair electrons have significant contribution to the birefringence,which is further determined by the corresponding density in the unit cell;about SHG response,the bismuth-oxygen polyhedra play a vital role.(2)We proposed a general strategy that introducing of hydrogen into the?-conjugated groups.The potential of[HCO₃]^-/[HBO₃]^2-/[HC₃N₃O₃]^2-groups as excellent birefringent chromophores has been evaluated through the quantum chemistry calculation.Compared to the traditional[CO₃]^2-/[BO₃]^3-/[C₃N₃O₃]^3-groups,they show similar HOMO-LUMO energy gaps and greater polarizability anisotropy.A₂CO₃,AHCO₃(A=Na,K,Rb,Cs),K₃CO₃F and K₂HCO₃F·H₂O were screened out and used as structural templates,it is found that compounds with[HCO₃]^-unit have larger band gap and birefringence than compounds with[CO₃]^2-unit.Consistent results were also found in Na₃BO₃/Ca₃(C₃N₃O₃)₂ to Na₂HBO₃/Rb₂(HC₃N₃O₃).To further clarify the origin of the enhancement in band gaps and birefringence,the bonding electron density difference??and electron density difference were analyzed.It is indicated the enhanced optical anisotropy can be attributed to the“tail effect”of the extended electronic distribution from[CO₃]^2-/[BO₃]^3-/[C₃N₃O₃]^3-groups to[HCO₃]^-/[HBO₃]^2-/[HC₃N₃O₃]^2-groups.This discovery is conducive to explore large birefringent materials in the system with[HCO₃]^-/[HBO₃]^2-/[HC₃N₃O₃]^2-groups.Simultaneously,through comparative analysis,the optical performance of Pb CO₃ and Bi BO₃ are also studied,indicating that introducing cations with stereochemically active lone pair electrons is beneficial to enhance the birefringence of compounds.