Growth, characterization and thermal behaviour of gel grown mixed Gd–Sr–molybdate crystals

The growth of mixed Gd–Sr–molybdate crystals, bearing stoichiometric composition Gd2Sr7Mo9O37·14H2O, is achieved by gel encapsulation technique, using the system RCl3(NH4)6Mo7O24NH4NO3HNO3Na2SiO3 (where R=Gd:Sr). Addition of Sr in the lattice of pure Gd-heptamolybdate (reported elsewhere) influences the morphological development of grown crystals. While pure Gd-heptamolybdate assumes spherulitic morphology under all conditions of growth, addition of Sr modifies the morphology of mixed crystals drastically and develops them as agglomerations of stacked hexagonal crystal platelets. The mixed crystals carry water of hydration and are thermally unstable. Various physico-chemical techniques, which include optical and scanning electron microscopy/XRD/energy dispersive analysis of X-rays and FT-IR were used to characterize the grown crystals. Thermal behaviour of these crystals using the thermoanalytical techniques, including TG/DTA and DSC is discussed. Thermal analysis suggests decomposition of the material in different temperature intervals. Results obtained on application of TG based models viz., Horowitz–Metzger, Coats–Redfern and Piloyan–Novikova are reported. The results of the kinetics of thermal decomposition suggest contracting sphere model as the one that is relevant to the decomposition of mixed Gd–Sr–molybdate. The kinetic parameters, viz. the order of reaction, frequency factor and energy of activation using above-mentioned models, are computed.