Morphology-controlled synthesis of cubic cesium hydrogen silicododecatungstate crystals

Cubic particles of cesium hydrogen silicododecatungstate crystals are obtained for the first time by the use of spherical seed crystals and control of the Cs⁺ to SiW₁₂O₄₀^4- (Cs/POM) ratio in the synthetic solution. The morphology of the particles is controlled between rhombic dodecahedra faceted with {110} planes (thermodynamically stable morphology) and cubes faceted with {100} planes. Scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy analysis of the cross-section shows that the cubes possess a core-shell structure, and the Cs/POM ratio of the shell (ave. 3.24) is larger than that of the core (ave. 2.76), suggesting the existence of anion (POM) vacancies in the shell. Solid state magic angle spinning NMR spectroscopy, nitrogen adsorption, and water vapor sorption measurements of the cubes show that the porous core is covered by the dense shell, and only water molecules can diffuse through the dense shell via the anion vacancies. Despite the small amounts of acidic protons, the cubes exhibit moderate proton conductivity (2.5 × 10^-4 S cm^-1) at room temperature under water vapor (298 K, P/P₀ = 0.95), suggesting that mobile water molecules in the anion vacancies contribute to the proton conduction.