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.