Thermal Degradation of Polyoxotungstates—An Effective Method for the Preparation of Tungsten Bronzes

Abstract

The thermal degradation of ammonium salts of five polyoxotungstates, [AsIII12CeIII16(H2O)36W148O524]76− I, [AsIII5CeIII4 (H2O)4W39O143]25− II, [AsIII3(UVIO2)3(H2O)6W30O105]15− III, [(W5O18)2CeIII]9− IV, and [(W5O18)2ThIV]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) CexWO3, x=0.066–0.162 with lattice parameters ranging from a=3.820(3) to 3.836(3) Å, of thorium(IV) ThxWO3, x∼0.1 with a=3.828(1) Å, and of uranium(IV) UxWO3, 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.