Studies on synthetic galloalunites AGa3(SO4)2(OH)6: Synthesis, thermal analysis, and X-ray characterization

Stoichiometric end member galloalunites of the general formula AGa3(SO4)2(OH)6, with A = Na+, K+, Rb+, H3O+, and NH4+ have been synthesized under hydrothermal conditions. These galloalunites were characterized by chemical methods, thermal analysis (DSC, TG coupled with mass spectroscopy), and powder X-ray diffraction (XRD). The stages of thermal decomposition of sodium, potassium and rubidium galloalunite show a common decomposition mechanism forming β-Ga2O3 and A2SO4 (A = Na+, K+, and Rb+) while ammonium and oxonium galloalunite decompose under formation of pure β-Ga2O3. The thermogravimetric results confirmed the analytical results on the galloalunites and thereby verified the stoichiometry of these synthetic products. Galloalunites with different monovalent cations in A site (i.e. Na+, K+, Rb+, H3O+ and NH4+) crystallize in the rhombohedral space group image (#166). The effects of substitution on the unit cell parameters are rationalized in terms of the structural arrangements in galloalunites. The unit cell parameter c increases with increasing effective ionic radii of the cation in the A site, whereas the parameter a changes to a much lesser degree.