Pure samples of a series of yttrium selenites, AY(SeO3)2 (A = Na, K, Rb, and Cs), were prepared through hydrothermal reactions using A2CO3, Y(NO3)3·6H2O, and SeO2. All four materials have 3D framework structures; however, they exhibit different channel geometries. The three-dimensional framework of NaY(SeO3)2 consists of zigzag chains of vertex-shared YO7 polyhedra and SeO3 linkers. KY(SeO3)2 and RbY(SeO3)2 reveal channels composed of corner-shared YO6 octahedra and SeO3 polyhedra. CsY(SeO3)2 exhibits a highly symmetric cage structure with radially spreading linkages of YO6 and SeO3 groups. Close structural examinations suggest that the cation size and coordination environment significantly influence the backbone architectures and their centricities. Second-harmonic generation (SHG) measurements on the powder sample of NaY(SeO3)2 reveal that the NCS selenite possesses a similar SHG efficiency to that of NH4H2PO4 (ADP).

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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 d(0) cation (Mo(6+)) and the lone pair cation (Se(4+)). 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.

Single crystals of the title compounds are hydrothermally prepared from aqueous mixtures of Y(NO3)3, M2CO3 (M: Na, K, Rb, and Cs), and SeO2 (autoclave, 230 °C, 4 d, 98, 99, 99, and 90% yield for Na, K, Rb, and Cs compounds, resp.).

The new compounds (IVa,b) and (VI) are characterized by powder and single crystal XRD, IR and UV/VIS diffuse reflectance spectroscopy, thermogravimetric analyses, ion‐exchange experiments, and SEM/EDX.