Synthesis and thermal behaviour of gallium-substituted aluminosilicate inorganic polymers

Abstract

A combined sol–gel and solid-state method reported for the synthesis of gallium silicate analogues of aluminosilicate inorganic polymers has also been extended to the formation of related compounds with a range of Al-for Ga substitutions. Homogeneous, robust products were obtained at an optimum composition of SiO₂:(Ga₂O₃ + Al₂O₃) = 7. After curing at 40 °C, all the products were typically X-ray amorphous, and the Al and Ga was shown by ²⁷Al and ⁷¹Ga MAS NMR spectroscopy to be in solely tetrahedral coordination. The ²⁹Si MAS NMR spectra were as expected for silicate inorganic polymers, but also indicated the presence of some unreacted silica. Electron microscopy in conjunction with EDS elemental mapping showed that the Ga, Al and Si was homogeneously distributed in the products. Thermal treatment of these compounds results in endothermic water loss at about 75–160 °C followed by an exothermic event at about 950 °C corresponding to crystallization of KGaSi₂O₆ in the gallium end-member. By contrast, the Al-substituted compounds never fully crystallised, but melted at 1200 °C to an X-ray amorphous product.