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

Abstract

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.