Rb₃MgB₅O₁₀ and LiBaAl(BO₃)₂: covalent tetrahedra MO₄-containing borates with deep-ultraviolet cutoff edges

Borates are favored by materials scientists and chemists because of the significant electronegativity difference between B and O atoms and their flexible assembly modes resulting in abundant structures and excellent properties. For the design of deep-ultraviolet (DUV) optical crystals with excellent macroscopic performance, it is crucial to choose appropriate cations and anionic groups and microscopically reasonable assembly patterns. Herein, by introducing covalent tetrahedra ([MO₄], M = Mg, Al), two new mixed alkali metal and alkaline earth metal borates, Rb₃MgB₅O₁₀ and LiBaAl(BO₃)₂, were synthesized using the melt method and high-temperature solution method. They contain M-B-O two-dimensional (2D) layers (2∞[MgB₅O₁₀] and 2∞[Al(BO₃)₂], respectively) composed of isolated B-O groups ([B₅O₁₀]^5- and [BO₃]^3-, respectively) and metal-centered tetrahedral connectors ([MgO₄]^6- and [AlO₄]^5-, respectively). Combining experiments and theoretical calculations shows that the two compounds have short cutoff edges (<200 nm) and moderate birefringences. Further analysis manifests that the isolated [MO₄] covalent tetrahedra can optimize the arrangement of anion groups, guarantee the balanced optical properties of materials, and point out the direction for further exploration of novel borate structures.