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Isomorphous framework cation substitution in the alumosilicatesodalites: synthesis, structural and spectroscopic studies of nitritecontaining phases

Abstract Partial substitution of aluminium with gallium in the Na8[Al1–yGaySiO4]6(NO2)2 sodalite system (y = 0–1) revealed an extensive isomorphous miscibility. Each composition (y = 0–1) is isotypic to either the alumosilicate or gallosilicate end members (y = 0 or 1, respectively). The Al/Ga ratio was controlled by selective initial stoichiometries in the direct hydrothermal syntheses. The samples were investigated by X-ray powder diffraction, SEM, FTIR and 29Si MAS NMR spectroscopies. The gallium concentration in the framework was calculated using the X-ray data Rietveld refinement, which was always lower than the feed concentration in each composition. The cell parameter and tilt angle increased, while the T—O—T angle and six-ring window dimension decreased with increasing gallium content in the framework. The Al/Ga—O distance is the mean of Al—O and Ga—O distances and increases with increasing gallium content. In contrast, the Si—O distance fluctuates close to 163 pm with increasing gallium insertion. The broadening of the XRD peaks in the patterns with mid-range gallium concentrations (y = 0.35–0.70) is associated with the formation of Al-rich and Ga-rich domains, which is explained in terms of Al/Ga—O distances and average crystal size with respect to Al/Ga ratio in the framework. The domains are assumed to be smaller than the coherence length of the X-ray radiation but large enough to detect by MAS NMR techniques.