Structural Insights into Borates with an Anion-Templated Open-Framework Configuration: Asymmetric K2BaB16O26 versus Centrosymmetric K3CsB20O32 and Na2M2NB18O30 (M=Rb, Cs; N=Ba, Pb)

Five new mixed-metal borate crystals, namely, K₂BaB₁₆O₂₆, K₃CsB₂₀O₃₂, Na₂Cs₂BaB₁₈O₃₀, Na₂Rb₂PbB₁₈O₃₀, and Na₂Cs₂PbB₁₈O₃₀, 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 [B₅O₁₀]⁵⁻ and [B₃O₇]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. [ABSTRACT FROM AUTHOR]