分子结构设计是开发新化合物和通过原子尺度操纵优化晶体结构的一种引人注目的策略. 在这个工作中, 利用分子工程的思想, 基于SBBO结构, 成功设计并合成两个新型氟碳酸盐KMgLi₂(CO₃)₂F和RbMgLi₂(CO₃)₂F. 在两个结构中, a-b平面是由CO₃和LiO₃F阴离子基团组成的无限[Li₃C₃O₆F₃]_∞层, 进一步相邻的层通过F原子连接形成一个独特的[Li₆C₆O₁₂F₃]_∞双层. 这种结构特征对改善晶体的层状生长习性和消除晶体的多晶性有很大的帮助. 光学测试表明, 该系列晶体具有大的双折射和短的紫外截止边, 是深紫外双折射晶体良好的候选材料.

Molecular engineering design is an effective strategy to develop new compounds and optimize the crystal structure by atomic-scale manipulation. In this work, a novel series of fluoride carbonates AMgLi₂(CO₃)₂F (A=K, Rb) are rationally developed by taking Sr₂Be₂B₂O₇ (SBBO) as the parent compound. Their crystal structures are established by single crystal X-ray diffraction. KMgLi₂(CO₃)₂F belongs to centrosymmetric hexagonal system and crystallizes in P6₃/m with a=b=0.4775 nm, c=1.4782 nm. RbMgLi₂(CO₃)₂F crystallizes in the centrosymmetric trigonal space group P3̅1c with a=b=0.4787 nm, c=1.4966 nm. In all of their structures, the a-b plane is the infinite lattice layer [Li₃C₃O₆F₃]_∞ made up of [CO₃] and [LiO₃F] anionic groups. The adjacent layers are further connected with fluorine bridge atoms to form [Li₆C₆O₁₂F₃]_∞ double layers. In the structure of KMgLi₂(CO₃)₂F, all the [CO₃] groups not only parallel to a-b plane but also align in the exactly same orientation in the same double layers. While the [CO₃] groups in RbMgLi₂(CO₃)₂F are parallel to a-b plane but align in the different orientation. But anyway, their structural characteristic is greatly beneficial to improve the layering-growth habit and eliminate polymorphism of a crystal. Since AMgLi₂(CO₃)₂F (A=K, Rb) inherit the beneficial layered structure of SBBO, AMgLi₂(CO₃)₂F (A=K, Rb) should have superior optical properties including short ultraviolet (UV) absorption edges and large birefringences. In order to confirm inference, ultraviolet-visible diffuse reflectance spectroscopy data were recorded at room temperature using a powder sample with BaSO₄ as a standard (100% reflectance) on a PerkinElmer Lambda-950 ultraviolet visible-near infrared spectrophotometer over the scan range 200—2500 nm. The results showed KMgLi₂(CO₃)₂F and RbMgLi₂(CO₃)₂F exhibited a wide transparency window, >80%, from 200 to 2500 nm, indicating that both of them had short UV cutoff edges below 200 nm. The birefringent values of the titled compounds were measured on a Nikon ECLIPSE LV100 POL polarizing microscope, which revealed that KMgLi₂(CO₃)₂F and RbMgLi₂(CO₃)₂F had large birefringences (0.111@546.1 nm for KMgLi₂(CO₃)₂F and 0.113@546.1 nm for RbMgLi₂(CO₃)₂F, respectively). In addition, theoretical cal-culations on electronic structure were carried out to explain the experimental results. Our preliminary results indicate that both compounds have promising applications as deep-UV birefringent materials.