Structural Design And Characterization Of Nonlinear Optical Crystals Assisted By Octahedral Groups

Nonlinear optical crystals are the most important part for widening the wavelength of solid-state lasers,and their development is closely related to the development of laser technology.To design and synthesize a new nonlinear optical crystal,it is necessary to pay attention to the types of structure groups,what's more,control their arrangements to make the microscopic dipoles be effectively superimposed in one structure,which would result a large nonlinear optical response.According to the anionic group theory,planar B-O groups with?-conjugation and IO₃groups with lone pairs are preferred options to design nonlinear optical crystals with large second harmonic generations(SHG),but how to make these groups in a well-organized arrangement is still difficult.The MO₆(M=metal cations)octahedron has four coplanar equatorial sites,of which can regulate the anionic groups in the same plane;and two axial positions,of which can be used as nodes to connect the whole structure.Based on the above ideas,we mainly carried out our work in two aspects:1.Synthesis and characterization of the first pyro-borate Ba₄Ca(B₂O₅)₂F₂with a large SHG response regulated by Ca O₆ octahedronBorates with?-conjugation have important applications in ultraviolet and deep-ultraviolet nonlinear optics.To obtain materials with a wide transmission range,alkali metal and alkaline earth metal cations without d-d,and f-f electronic transitions are the first choice,among which Ca²⁺cations are easier to form Ca O₆ octahedron attracted our attention.Therefore,the design and synthesis of borates containing Ca O₆octahedron were carried out,and successfully obtained Ba₄Ca(B₂O₅)₂F₂.The property measurements indicate that it is 2.2 times that of KH₂PO₄(KDP),and the UV cut-off edge is less than 190 nm.The easily distorted B₂O₅ groups in the Ca-B-O layer adopt coplanar arrangements,which are favorable for the formation of?-conjugation.Structure-property relationship indicates that such layer provides an ideal template for designing pyro-borate nonlinear optical crystals with excellent optical properties.2.Synthesis and characterization of K₂Zn(IO₃)₃(I₂O₆H)(IO₃H)(H₂O)with poly-iodate regulated by Zn O₆ octahedronIodates with stereochemical activity of lone pairs are excellent systems for designing nonlinear optical materials with large SHG responses.To further enhance the SHG response of the material,previous studies mainly focus on d~0 transition metals.Based on the requirements of materials for SHG response and band gap,we choose d¹⁰ transition metal Zn²⁺,which could exhibit distorted Zn O₆ octahedron,to design and synthesize the new structure.We successfully obtained K₂Zn(IO₃)₃(I₂O₆H)(IO₃H)(H₂O)by hydrothermal synthesis.In the Zn-I-O layer,the IO₃ groups arrange parallelly and in the same direction,which contributes a large polarity.The performance measurements show that the compound exhibits a large SHG response(12 times that of KDP),a large band gap(4.0 e V),and a moderate birefringence(0.044@1064 nm).This provides a new idea for the exploration of new iodates with both large SHG response and large band gap.3.Synthesis and characterization of other new borate compoundsIn the above researches,we also obtained other new crystals,such as Ba₂Zn₂B₆O₁₃ and Ba₂BO₃Cl.Among them,Ba₂Zn₂B₆O₁₃ crystallizes in the non-centrosymmetric space group Cc,which can exhibit nonlinear optical response.Also,theoretical calculations show that it has a large birefringence(0.0653@1064nm),which indicates that it is a potential ultraviolet nonlinear optical crystal.