Rb 3 MgB 5 O 10 and LiBaAl(BO 3 ) 2 : covalent tetrahedra MO 4 -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 4 ], M = Mg, Al), two new mixed alkali metal and alkaline earth metal borates, Rb 3 MgB 5 O 10 and LiBaAl(BO 3 ) 2 , were synthesized using the melt method and high-temperature solution method. They contain M-B-O two-dimensional (2D) layers (2∞[MgB 5 O 10 ] and 2∞[Al(BO 3 ) 2 ], respectively) composed of isolated B-O groups ([B 5 O 10 ] 5- and [BO 3 ] 3- , respectively) and metal-centered tetrahedral connectors ([MgO 4 ] 6- and [AlO 4 ] 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 [MO 4 ] 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.