Effects of the Orientation of [B5O11]7– Fundamental Building Blocks on Layered Structures Based on the Pentaborates

Four new layered pentaborates Rb4Ba2.5­B20O34­Br, Rb2Ba4­B20O34­Br2, Rb4Ba2.5­B20O34­Cl, and KBa­B5O9 have been successfully synthesized via a high-temperature solution method; the former three are the first series of compounds reported in the Rb-Ba-B-O-X (X = halogen) system. Interestingly, the structures of the above compounds are composed of the same [B5­O11]7– fundamental building block (FBB), which could be further linked to form 2∞[B10­O17]4– double layers for the former three compounds and 2∞[B5­O9]3– single layers for the last one. The structure comparisons among all the available anhydrous pentaborates reveal that the structures of the anionic framework are affected by the relationship between the orientation of [B5­O11]7– FBB axes and the layer plane (parallel or perpendicular), which will produce the different number of terminal O atoms in the initial pentaborate blocks. The viewpoints give us a feasible way to investigate the layered structures and expand the structural diversity of borates. Furthermore, the infrared spectra, UV–vis–NIR diffuse reflectance spectra, and thermal behaviors of these compounds were also studied.