Structure-Directing Effect of Alkali Metal Cations in New Molybdenum Selenites, Na₂Mo₂O₅(SeO₃)₂, K₂Mo₂O₅(SeO₃)₂, and Rb₂Mo₃O₇(SeO₃)₃

Abstract

Both single crystals and pure polycrystalline samples of three new quaternary alkali metal molybdenum selenites, Na₂Mo₂O₅(SeO₃)₂, K₂Mo₂O₅(SeO₃)₂, and Rb₂Mo₃O₇(SeO₃)₃, have been synthesized through hydrothermal and solid-state reactions using A₂CO₃ (A = Na, K, and Rb), MoO₃, and SeO₂ as reagents. The frameworks of all three materials consist of both families of second-order Jahn–Teller distortive cations, i.e., the d⁰ cation (Mo⁶⁺) and the lone pair cation (Se⁴⁺). Although the extent of framework distortions and the resulting occupation sites of alkali metal cations are dissimilar, Na₂Mo₂O₅(SeO₃)₂ and K₂Mo₂O₅(SeO₃)₂ exhibit similar three-dimensional networks that are composed of highly asymmetric Mo₂O₁₁ dimers and SeO₃ polyhedra. Rb₂Mo₃O₇(SeO₃)₃ reveals a two-dimensional structure that is built with Mo₃O₁₅ trimers and SeO₃ intralayer linkers. Close structural examinations suggest that the structure-directing effect of alkali metal cations is significant in determining the framework distortions and the dimensions of the molybdenum selenites. UV–vis diffuse reflectance and infrared spectroscopy, thermogravimetric analyses, and ion-exchange reactions are reported, as are out-of-center distortion and dipole moment calculations