Structural Insights into Borates with an Anion-Templated Open-Framework Configuration: Asymmetric K2 BaB16 O26 versus Centrosymmetric K3 CsB20 O32 and Na2 M2 NB18 O30 (M=Rb, Cs; N=Ba, Pb).

Five new mixed-metal borate crystals, namely, K2 BaB16 O26 , K3 CsB20 O32 , Na2 Cs2 BaB18 O30 , Na2 Rb2 PbB18 O30 , and Na2 Cs2 PbB18 O30 , have been synthesized by the high-temperature solution method. These structures were characterized by single-crystal X-ray diffraction and their B-O configurations are shown to feature complicated three-dimensional (3D) open-framework structures incorporating different types of channels. For all five compounds, the frameworks can be reduced to four-connected (4-c) uninodal nets with sra-, dia- and nbo-type topologies by assigning the [B5 O10 ]5- and [B3 O7 ]5- clusters as basic nodes. First-principles calculations have shown that the multiple cations in mixed-metal borates influence the ground-state electronic structures and associated optical absorptions. In addition, the crystal structures, thermal stabilities, and IR spectra of the title compounds have also been performed.