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 R3¯m (#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.

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► Stoichiometric galloalunites AGa3(SO4)2(OH)6, (A = Na+, K+, Rb+, H3O+, and NH4+).
► Phase pure synthesis under hydrothermal conditions.
► Stages of thermal decomposition of Na, K, Rb-galloalunite form Ga2O3 and A2SO4.
► Ammonium and oxonium galloalunite decompose under formation of pure β-Ga2O3.
► All galloalunites crystallize isomorphus in the rhombohedral space group R3¯m (#166).