A cation size effect on the framework structures in ABi2SeO3F5 (A = K and Rb): first examples of alkali metal bismuth selenite fluorides

Abstract

Two new centric alkali metal bismuth selenite fluorides, ABi₂SeO₃F₅ (A = K and Rb), have been synthesized through a soft hydrothermal method. KBi₂SeO₃F₅ crystallizes in an orthorhombic crystal system with the centric space group Pbcm possessing a three-dimensional (3D) network made up of SeO₃ trigonal pyramids and asymmetric BiO₃F₅ polyhedra. As regards compound RbBi₂SeO₃F₅, it crystallizes in a triclinic crystal system with a centric space group of P, which contains alternant two-dimensional (2D) [Bi₂SeO₃F₅]_∞¹⁻ layers separated by Rb⁺ cations. In terms of structure, the differences between KBi₂SeO₃F₅ and RbBi₂SeO₃F₅ originate from the size of alkali metal cations, which has a great influence on the framework geometry and the dimensionalities of crystals. The UV-vis diffuse reflectance spectroscopy study of powder samples indicates that the band gaps of KBi₂SeO₃F₅ and RbBi₂SeO₃F₅ are approximately 4.08 and 4.18 eV, respectively, which are the largest ones among the known bismuth selenites owing to the existence of alkali metal cations as well as fluorine anions in crystals. Furthermore, theoretical calculations show that the optical absorption of two compounds is mainly attributed to the contribution of BiO_xF_y polyhedra and [SeO₃]²⁻ anionic groups.