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.