Thermal degradation of polyoxotungstates - An effective method for the preparation of tungsten bronzes

Abstract

The thermal degradation of ammonium salts of five polyoxotungstates, [As12/(III)Ce16/(III)(H2O)36W148O524]76- I, [As5/(III)Ce4/(III) (H2O)4W39O143]25- II, [As3/(III) (U(VI)O2)3(H2O)6W30O105]15- III, [(W5O18)2Ce(III)]9- IV, and [(W5O18)2Th(IV)]8- V, in hydrogen up to 500°C and in argon or nitrogen up to 950°C was studied by means of thermal analysis (TG, DSC), X-ray diffraction methods, chemical analysis, and susceptibility measurements. X-ray analyses of the resulting deeply colored, arsenic-free powders, revealed the formation of cubic tungsten bronzes of cerium(III) Ce(x)WO3, x = 0.066-0.162 with lattice parameters ranging from a = 3.820(3) to 3.8360(3) Å, of thorium(IV) Th(x)WO3, x ~ 0.1 with a = 3.828(1) Å, and of uranium(IV) U(x)WO3, x ~ 0.1 with a = 3.797(1) Å as the main product in all experiments. These cubic bronzes were obtained in a one-step solid-state process at significantly lower temperatures than those reported previously by using classical methods of synthesis. (C) 2000 Academic Press.