M2Ca3B16O28 (M = Rb, Cs): structures analogous to SBBO with three-dimensional open-framework layers

Two isostructural alkali calcium borates, M2Ca3B16O28 (M = Rb, Cs) have been successfully synthesized by the flux method. The two compounds crystallize in the monoclinic space group C2/c (no. 15) and feature three-dimensional open-framework layers. The M+ (M = Rb, Cs) cations occupy in-layer three-dimensional micro-pores, and the Ca2+ cations bridge the layers to form the three-dimensional network. Based on the structure analysis, it is found that M2Ca3B16O28 (M = Rb, Cs) compounds have a similar structure to that of Sr2Be2B2O7. In addition, spectroscopic properties and first-principle calculations have been carried out on the title compounds.