RbMgBO and LiBaAl(BO): covalent tetrahedra MO-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 ([MO], M = Mg, Al), two new mixed alkali metal and alkaline earth metal borates, RbMgBO and LiBaAl(BO), were synthesized using the melt method and high-temperature solution method. They contain M-B-O two-dimensional (2D) layers (2∞[MgBO] and 2∞[Al(BO)], respectively) composed of isolated B-O groups ([BO] and [BO], respectively) and metal-centered tetrahedral connectors ([MgO] and [AlO], 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 [MO] 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.