Synthesis,Structure And Properties Of UV/DUV Alkali Earth Metal Borates And Rare Earth Borates

Borates are widely used in ultraviolet（UV）and deep ultraviolet（DUV）phototransistor materials because of their rich and flexible structure chemistry and excellent properties.In this thesis,borate crystal structure diversity and new UV or DUV crystal materials as the main research objectives,and through high temperature melt method and solid-state reaction method to explore new borate compounds.The main synthesis strategies are as follows:（1）by introducing some cations without d-d or f-f electron transitions,such as alkali metals,alkaline earth metals and some trivalent rare earth metals（Y,Sc,Gd,Lu,La）into the borate system to obtain compounds with short UV cutoff edges;（2）The crystal structure is adjusted by controlling the ratio of cations to B atoms.According to the above ideas and methods,five compounds were successfully obtained,and their crystal structures and properties were discussed in detail.In addition,the relationship between their structure and optical properties is analyzed by first principles.The main research results are as follows:1.Synthesis,structure and properties of a new borate Sr₃B₁₄O₂₄with rare infinite 2D double layer structureA new alkaline-earth metal borate,Sr₃B₁₄O₂₄,was successfully synthesized by high temperature solution method.It crystallizes in the monoclinal system P2₁/c（No.14）space group and features a fundamental building block（FBB）[B₁₄O₃₀]which is composed of eight[BO₃]and six[BO₄]units.The FBBs further condense to form an unwonted infinite 2D double layer extended in the bc plane,which greatly enriches the structural chemistry of borates.The UV-Vis-NIR diffuse reflectance spectrum shows that the cutoff edge of Sr₃B₁₄O₂₄ is less than 200 nm,which indicates its potential use in the deep UV region.Infrared spectrum and EDS element analysis also proved the rationality of the structure.In addition,in order to better understand the relationship between microstructure and macroscopic properties,the first-principles theoretical study was carried out.2.Synthesis,structure and properties of a new rare earth borate Na₂La₂B₁₀O₁₉A new complex alkali-rare earth metal borate Na₂La₂B₁₀O₁₉ was successfully synthesized by high temperature solution spontaneous crystallization.It crystallizes in the C2/c（No.15）space group of the centrosymmetric monoclinic system and has an infinite 2D double layer structure composed of[B₅O₁₂]FBB.It is based on the classical nonlinear optical crystal material La₂Ca B₁₀O₁₉ as a template,by replacing the alkali earth element Ca with the alkali metal element Na.Although they have a similar crystal structure,the arrangement between the two double layers is completely opposite,resulting in a structural shift.The UV cutoff of Na₂La₂B₁₀O₁₉ is less than 220 nm,and the birefringence of Na₂La₂B₁₀O₁₉ is 0.06@1064 nm according to first-principles calculation,which indicates that Na₂La₂B₁₀O₁₉ has potential as a UV birefringent crystal material.In addition,through REDA model analysis,the source of birefringence is mainly due to the contribution of B-O anionic group and La-O bond.3.Synthesis,structure and properties of three noncentrosymmetric polymetallic composite rare earth boratesThree non-centrosymmetric rare earth borates Rb₇Ca Y₂B₁₅O₃₀,Na Rb₆Ca Y₂B₁₅O₃₀ and Li Rb₆Ca Y₂B₁₅O₃₀ were successfully synthesized by spontaneous crystallization in high temperature solution method with the strategy of adjusting crystal structure by cation substitution.Both Rb₇Ca Y₂B₁₅O₃₀ and Na Rb₆Ca Y₂B₁₅O₃₀ crystallize in the non-centrosymmetric trigonal system R32 space group.However,Li Rb₆Ca Y₂B₁₅O₃₀ crystallizes in the non-centrosymmetric orthogonal system C222₁ space group,which is a new space group first appeared in the B₁₅O₃₀series of compounds.Their anion frames are 0D clusters of isolated[B₅O₁₀]units filled with cations to balance the charge and thus form a 3D monolithic structure.The UV cutoff edges of Rb₇Ca Y₂B₁₅O₃₀ and Na Rb₆Ca Y₂B₁₅O₃₀ are less than 195 nm and 185 nm,and the SHG effects are 1.2 and 1.4 times of KDP,respectively,with high thermal stability.These results indicate that they can be used as a potential UV nonlinear optical（NLO）crystal materials.In addition,the relationship between microstructure and macroscopic optical properties is discussed in detail through first-principles calculations.