The Design And Synthesis Of Alkaline-Earth Metal Borate With Isolated B-O Groups And Study On The Crystal Growth Of K₃B₆O₁₀Br

The borate crystals has wide variety of structure chemistry because the boron atoms can bond with oxygen atoms to form the BO₃ triangles or the BO₄ tetrahedra,then the BO₃triangles or the BO₄ tetrahedra via corner-or edge-sharing oxygen atoms to create various B-O clusters,resulting in special properties of compounds.In this paper,the research of new materials and crystal growth in borate system are mainly based on the following strategies:?a?Based on the anion group theory,coplanar or parallel alignment of isolated B-O groups are conducive to produce large optical anisotropy and polarizability to obtain large birefringence and nonlinear optical coefficient;?b?The introduction of cations with stereoactive lone pair electrons such as Pb²⁺into the structure can facilitate the generation of large birefringence index;?c?Compounds containing target functional units can be designed by adjusting the ratio of cations to boron;?d?Cosubstitution strategy can effectively guide the synthesis of new compounds and accelerate the development of new materials.The research results of new compounds and crystal growth are as follows:?1?Synthesis and characterization of birefringent crystal Pb₈Mg_xCa_1-x?BO₃?₆?x=0,0.4,1?Three new isostructural lead borates,Pb₈Mg_xCa_1-x?BO₃?₆?x=0,0.4,1?,have been synthesized via high temperature solution method.The single crystal structures of these three compounds contain_?[Pb₈B₆O₁₈]^2-double layers composed of planar BO₃ triangles,Pb?2?O₉and distorted Pb?1?O₅ polyhedra,these layers stack along c-axis,and the MgO₆(or CaO₆Mg_0.4Ca_0.6O₆)polyhedra fill into the layers to form the three-dimensional framework.Structural relationship analysis indicated that the twisted Pb?1?O₅ polyhedron and the nearly parallel BO₃ triangles maily contribute to the refractive index of the compound,and the First-principles calculations show that Pb₈Mg?BO₃?₆ and Pb₈Ca?BO₃?₆ have larger birefringence indices,which are 0.15 and 0.12@1064 nm,respectively.These three lead borates imply that they are potenticial birefringent materials for visible-near infrared applications.?2?Synthesis and Characterization of Li_3.366Mg_0.317B₂O₅The first pyroborate in the Li₂O-MgO-B₂O₃ system,Li_3.366Mg_0.317B₂O₅,has been synthesized via high temperature solution method.The crystal structure can be described as isolated B₂O₅ units connecting with the Li?2?O₅ chains to generate the_?[Li₂?2?B₂O₅]layers,which condense along a-axis,and the co-occupied Li?1?/Mg atoms fill into the voids between the layers to form the final 3D framework.Interestingly,edge-and face-sharing Li?2?O₅polyhedra are found in Li_3.366Mg_0.317B₂O₅,which configuration are rarely reported and only found in its maternal compound?-Li₄B₂O₅.In addition,the relationship between twist angle and dihedral angle between the two BO₃ triangles of B₂O₅ cluster in Li_4-2x-2x Mg_xB₂O₅?x=0,0.317,2?was also studied,and the effect of cationic Mg²⁺on the regulation of functional elements in its crystal structure was further discussed.?3?Crystal growth and optical properties characterization of nonlinear optical crystal K₃B₆O₁₀BrK₃B₆O₁₀Br crystal is a kind of nonlinear optical crystal with excellent properties,including wide transmission band,large nonlinear optical coefficient,suitable birefringence and good stability.It has important applications in the field of ultraviolet laser frequency conversion.However,the current flux PbO for K₃B₆O₁₀Br crystal growth will introduce the impurities in lattice,and cause serious absorption of the crystal in the ultraviolet region of 200-260 nm,which limits the application of K₃B₆O₁₀Br crystal in the ultraviolet region and reduces the laser damage threshold of the crystal.Therefore,the K₃B₆O₁₀Br crystal was explored by self-flux method,and the crystal growth technology was studied.Finally,the K₃B₆O₁₀Br crystals with high transmittance in the ultraviolet region were obtained.The transmission spectra show that the crystal has little absorption in 200-260 nm region.The experimental results indicate that the Pb²⁺ions in the flux system containing lead has negative effect on the ultraviolet transmittance.The change of refractive index and the phase matching angle at different temperature were measured and analyzed by prism coupling technique for the first time.The experimental results show that refractive index and phase matching angle of K₃B₆O₁₀Br crystal have slight changes when the temperature change from room temperature to 100 ^oC,which deduce K₃B₆O₁₀Br may obtain stable laser frequency doubling output.