Cubic cesium hydrogen silicododecatungstate with anisotropic morphology and polyoxometalate vacancies exhibiting selective water sorption and cation-exchange properties

A cesium hydrogen silicododecatungstate Cs_3.0H _0.3[SiW₁₂O₄₀]_0.83·3.0H ₂O (CsSiW) with a cubic cell and anion ([SiW₁₂O ₄₀]^4-) vacancies is synthesized, and the anion vacancies are utilized for selective water sorption and exchange of monovalent cations. Compound CsSiW consists of highly crystalline particles with a flat rhombic bipyramidal morphology, and the anisotropic morphology is reported for the first time among the cubic solid POM compounds. Due to the high crystallinity and flatness of CsSiW, direct atomic images (heavy cesium and tungsten atoms) are observed for the first time in solid POM compounds. The electron-microscopic images suggest that CsSiW grows through (i) formation of [Cs₃SiW ₁₂O₄₀]^- followed by adsorption of Cs ⁺, (ii) then parallel to the {110} plane, forming plates, and (iii) perpendicular to the {110} plane accompanied by the generation of anion vacancies. The mismatch between anion to cation site ratio (1:3) and cation to anion charge ratio (1:4) would be crucial for the anisotropic crystal growth and generation of anion vacancies.