碱土金属卤素硼酸盐紫外倍频晶体的结构设计、水热合成和性质研究

Studies on the ultraviolet (UV) frequency doubling crystals are of importance because they are the key materials of the all-solid state laser and play crucial roles in the application and development of the UV laser technology. However, it is still a challenge to design and synthesize UV frequency doubling crystals with excellent comprehensive properties such as large second harmonic generation, wide transparency range, stable physical and chemical properties, and so on. This project intends to explore novel UV frequency doubling crystals in tne alkaline-earth halogen borates by considering the following merits. Firstly, the borates possess versatile structures and both of the BO3 groups and the polyanion groups consisting of BO3 groups have nonlinear optical (NLO) activities. Secondly, the chemical bond formed with the alkaline-earth element is ionic and there is no d-d electron transition, which are conducive to obtain crystals with large band gap and UV transparency. Thirdly, halogen atoms are electronegative elements which can greatly influence the electron distribution in the structure and thus help to fabricate acentric structures. Therefore, this project focus on the alkaline-earth halogen borate systems. Because the radius values, valence states and bonding types of OH- and halogen anions are similar, they can be replaced by each other. Therefore, we try to introduce the halogen anions into the acentric crystal lattice of the hydrate alkaline-earth borate obtained in our previous work. By hydrothermal method, we try to tune the SHG properties of the crystal by tuning the alkaline-earth cations and the halogen anions, and synthesize novel UV frequency doubling crystals of alkaline-earth halogen borates. Through analyzing the structural characters and the microscopic electronic structure properties, we try to explore the influence of the crystal composition and structure upon the macroscopic nonlinear optical properties. This project will shed some light on the structural design and controlled synthesis of novel UV nonlinear optical materials.

紫外倍频晶体是全固态激光器的关键材料，其研制对推动紫外激光技术的应用和发展具有重要意义。设计合成倍频效应大、透光波段宽、物化性能稳定等综合性能优异的紫外倍频晶体仍是一项挑战。本项目拟结合以下三点："硼酸盐结构复杂多样且BO3基团及其组成的硼氧聚阴离子基团具有ＮＬＯ活性；碱土元素无d-d电子跃迁，形成的键离子性强，利于产物的紫外光透过和带隙增大；卤素原子电负性大，能够较大程度影响电子分布，辅助形成不对称中心结构"，在碱土-卤素-硼酸盐体系探索新型紫外倍频晶体。以无心水合碱土金属硼酸盐结构为切入点，利用OH-与卤素离子在半径、价态和成键类型方面的相似性，在结构中引入卤素离子。采用水热法调控碱土和卤素离子来影响晶体组成和结构，进而调控晶体倍频效应，合成倍频效应大、紫外吸收边短的倍频晶体。分析晶体结构和电子结构特征，揭示组成和结构对倍频效应的影响规律，为新型紫外倍频晶体结构设计和可控合成提供指导。

紫外倍频晶体是全固态激光器的关键材料，深入开展紫外倍频晶体的合成、结构、性质、以及结构-性能关系等基础研究，对丰富非线性光学材料的理论体系、开发新型性能优良的紫外倍频晶体、推动固体激光技术在紫外/深紫外区的应用和发展均具有重要意义。.本项目旨在碱土金属卤素硼酸盐体系设计合成新型紫外倍频晶体，研究晶体结构、电子结构和物化性能，揭示组成和结构对倍频效应的影响规律，为新型紫外倍频晶体结构设计和可控合成提供指导。.通过项目实施，制备获得了五种紫外非线性光学晶体（紫外截止边均低于190 nm），Sr4B10O18(OH)2·2H2O和 Sr2B5O9(OH)·H2O倍频效应是KDP的2倍和3倍， 具有同构构型的Ba4(BO3)3(SiO4)·Ba3X (X = Cl， Br)倍频效应与KDP相当，这三种化合物能实现1064nm基频波与倍频波的相位匹配，是潜在的紫外倍频晶体材料。Ba2B5O9Cl·0.5H2O晶体的粉末倍频效应与KDP相当，不能实现1064 nm基频波与倍频波的相位匹配。.通过结构-性能关系研究表明，Sr4B10O18(OH)2·2H2O 和Sr2B5O9(OH)·H2O倍频效应主要来自于B-O基团的贡献，同时碱土金属阳离子和O-H基团的贡献也不能忽略，因此碱土金属阳离子和O-H基团也有可能成为非线性光学材料结构设计的功能基团；采用键价法进行了偶极矩分析, Ba2B5O9Cl·0.5H2O单胞中沿c轴方向的净偶极矩主要来自于Ba1O8Cl2配位多面体和B1O3三角形的贡献，氢键的形成可能削弱了晶体的极性。以XBa6极性离子盐单元作为极性模板形成了Ba4(BO3)3(SiO4)·Ba3X是离子盐化合物，极性单元的引入对形成不对称中心的结构有利，该研究工作丰富了非线性光学材料的研究体系，为在卤素硼硅酸盐体系设计和合成新型紫外非线性光学材料带来新思路。