Alkali earth MOx (x = 6, 7, 9, 12) polyhedra tuned cadmium selenites with different dimensions and diverse SeO32− coordinations

Abstract

A series of alkali earth cadmium selenites including MgCd₃(SeO₃)₄ (1), Ca_0.7Cd_2.3(SeO₃)₃ (2), SrCd(SeO₃)₂ (3) and BaCd(SeO₃)₂ (4) are synthesized by hydrothermal reaction and studied in detail by single crystal and powdered X-ray diffractions, thermal analyses, IR and UV spectral measurements and dipole moment calculation. 1 features a 3D anionic [Cd₃(SeO₃)₄]²⁻ framework constructed from isolated SeO₃ trigonal pyramids and 2D [Cd₃O₁₂]_n layers, in which the [Cd₃O₁₂]_n layer is composed of edge-shared [Cd₂O₁₀]¹⁶⁻ dimers and butterfly-like [Cd₄O₂₀]³²⁻ tetramers. 2 shows a smooth 2D [Cd₂(SeO₃)₃]_n layer composed of unique [Cd(SeO₃)]_n double chains. 3 exhibits a wavelike 2D [Cd(SeO₃)₂]_n layer constructed from corner-shared CdO₆ octahedra and isolated SeO₃ trigonal pyramids. 4 displays a windmill-like 1D [Cd(SeO₃)₂]_n chain built from zigzag [CdO₄]_n chain motifs and isolated SeO₃ trigonal pyramids. The alkali earth oxide motifs demonstrate interesting and regular changes from isolated MgO₆ octahedra, CaO₇ polyhedra, and SrO₉ tri-capped trigonal prisms based on a ribbon chain to BaO₁₂ icosahedra based on a brucite-like layer, and meanwhile the SeO₃ groups are observed from variable tetradentate to hexadentate coordination modes. These flexible motifs reveal that cadmium selenites are strongly affected by the size of the alkali earth cations, providing a feasible approach to modulating their space structure via selection of template cation.