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.

Key words: structure design, deep UV, fluoride carbonate, birefringent crystal