Three Mixed-Alkaline Borates: Na2M2B20O32 (M = Rb, Cs) with Two Interpenetrating Three-Dimensional B-O Networks and Li4Cs4B40O64 with Fundamental Building Block B40O77

Three new mixed-alkaline borates Na2M2B20O32 (M = Rb, Cs) and Li4Cs4B40O64 with unique structures were synthesized by the spontaneous nucleation method, and their structures were determined by single-crystal X-ray diffraction. The B-O networks of Na2M2B20O32 (M = Rb, Cs) are constructed by two independent interpenetrating three-dimensional (3D) frameworks, which is the first case in anhydrous mixed-alkaline borate systems. In addition, Li4Cs4B40O64 shows high B-O polymerization with a new fundamental building block B40O77. Meanwhile, detailed structure comparisons containing the cation effect on the framework have been discussed. UV–vis–NIR diffuse reflectance spectra and the infrared spectra were measured. The band structures and the density of states were performed using density functional theory calculation.