Effect of Halogen (Cl, Br) on the Symmetry of Flexible Perovskite-Related Framework

The perovskite structure is a good candidate for the design of functional materials. On the basis of the combination of B₆O₁₃ groups and XM₆ (X = Cl, Br; M = alkali metals) octahedra, three new perovskite-related crystals Na₃B₆O₁₀Cl, RbNa₂B₆O₁₀Cl, and RbNa₂B₆O₁₀Br have been synthesized by a high-temperature solution method for the first time. Na₃B₆O₁₀Cl and RbNa₂B₆O₁₀Cl are isostructural and crystallize in the noncentrosymmetric (NCS) space group P2₁2₁2₁ (No. 19), while RbNa₂B₆O₁₀Br belongs to the centrosymmetric (CS) space group Pnma (No. 62). The phenomenon that Cl-containing borates are not isostructural with corresponding Br-containing borates is extremely rare among borates. Detailed structure analysis suggests that the difference is owing to the effect of the halogen (Cl, Br) on the symmetry of the flexible perovskite-related framework. In addition, thermal analyses, IR spectroscopy, the UV–vis–NIR diffuse reflectance spectrum, and first-principles theoretical studies have also been performed on the three compounds.