Mg assists in modulating the dimensionalities of the anionic frameworks of borates.

Three Mg-containing borates were obtained by high-temperature spontaneous crystallization. In the (A 2 O)- or (A 2 O-MO)-MgO-B 2 O 3  system (A is alkali metal and M is alkaline-earth metal) reported in the ICSD, Li 4 Mg 3 SrB 12 O 24 is the first compound that contains one-dimensional infinite anionic chains, and the two examples of the isostructural A 2 Mg 3 B 16 O 28 (A = Rb, Cs) exhibit a two-dimensional infinite bilayer structure for the first time, which contributes to the enrichment of the structural chemistry of Mg-containing borates. Besides, the results of comparison and analysis in this system clearly show that Mg not only affects the anionic frameworks of borates to produce low-dimensional structures but, together with the ratio of N cation / N B , is responsible for the dimensionalities of the anionic frameworks in borates. The optical properties of the three compounds also show that they all have short cutoff edges, and Cs 2 Mg 3 B 16 O 28 , in particular, could reach the deep-ultraviolet region (<200 nm).