Cation Size Effect on the Framework Structures in a Series of New Alkali-Metal Indium Selenites, AIn(SeO₃)₂ (A = Na, K, Rb, and Cs)

A new family of quaternary alkali-metal indium selenites, AIn­(SeO₃)₂ (A = Na, K, Rb, and Cs) have been synthesized, as crystals and pure polycrystalline phases through standard solid-state and hydrothermal reactions. The structures of the reported materials have been determined by single-crystal X-ray diffraction. While AIn­(SeO₃)₂ (A = Na, K, and Rb) crystallize in the orthorhombic space group, Pnma, with three-dimensional framework structures, CsIn­(SeO₃)₂ crystallizes in the trigonal space group, R3̅m, with a two-dimensional structure. All of the reported materials, however, share a common structural motif, a network of corner-shared InO₆ octahedra and SeO₃ groups. Interestingly, the size of the alkali-metal cations profoundly influences the bonding nature of the SeO₃ group to the InO₆ octahedra. Complete characterizations including infrared spectroscopy, elemental analyses, and thermal analyses for the compounds are also presented, as are dipole moment calculations. A detailed cation size effect on the framework structure is discussed.