Isomorphous gallium substitution in the alumosilicate sodalite framework: synthesis and structural studies of chloride and bromide containing phases

Abstract

In the Na8[T1T2O4]6Y2sodalite system (Y= Cl, Br,T2= Si) the aluminium bearingT1position was successively substituted with gallium. The synthesized products represent the new composition Na8[Al1–yGay-SiO4]6Y2(0 ≤y≤ 1), where the Al/Ga ratio is calculated from the X-ray powder data Rietveld refinements. In each composition, the gallium concentration in the framework is observed lower than that in the initial feed. The linear expansion of the lattice parameters has been observed from [AlSiO4]6to [GaSiO4]6in increasing gallium concentration in the framework. The position of gallium and aluminium has been refined on a single crystallographic site. This result also indicates that both aluminium and gallium are indistinguishable and statistically distributed in theT1-sites. TheT1-O distance, therefore, offers an averaged magnitude of Al/Ga—O distance which increases with increasing gallium concentration. In contrast, the Si—O distance remains almost constant at 162.8 pm (2σ) and 163.4 pm (3σ) for chloride and bromide sodalites, respectively. The increasing average tilt of the (Al,Ga)O4and SiO4tetrahedra leads to a decrease of the six-ring window dimension as a consequence of increasing gallium incorporation in the framework. The simultaneous shortening of Na—O and Na—Cl/Br distances, with increasing gallium content, shows a direct interaction between the framework host and the enclathrated guest. Additionally, the tetragonal tetrahedron distortion and the framework oxygens-hybridization are discussed as a function of gallium concentration in the two new sodalite series.