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

Abstract

Both\nsingle crystals and pure polycrystalline samples of three new quaternary\nalkali metal molybdenum selenites, Na₂Mo₂O₅(SeO₃)₂, K₂Mo₂O₅(SeO₃)₂, and Rb₂Mo₃O₇(SeO₃)₃, have been synthesized\nthrough hydrothermal and solid-state reactions using A₂CO₃ (A = Na, K, and Rb), MoO₃, and SeO₂ as reagents. The frameworks of all three materials consist\nof both families of second-order Jahn–Teller distortive cations,\ni.e., the d⁰ cation (Mo⁶⁺) and the lone pair\ncation (Se⁴⁺). Although the extent of framework distortions\nand the resulting occupation sites of alkali metal cations are dissimilar,\nNa₂Mo₂O₅(SeO₃)₂ and K₂Mo₂O₅(SeO₃)₂ exhibit similar three-dimensional networks that are composed\nof highly asymmetric Mo₂O₁₁ dimers and SeO₃ polyhedra. Rb₂Mo₃O₇(SeO₃)₃ reveals a two-dimensional structure that is\nbuilt with Mo₃O₁₅ trimers and SeO₃ intralayer linkers. Close structural examinations suggest that the\nstructure-directing effect of alkali metal cations is significant\nin determining the framework distortions and the dimensions of the\nmolybdenum selenites. UV–vis diffuse reflectance and infrared\nspectroscopy, thermogravimetric analyses, and ion-exchange reactions\nare reported, as are out-of-center distortion and dipole moment calculations.