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

Abstract The thermal degradation of ammonium salts of five polyoxotungstates, [As III 12 Ce III 16 (H 2 O) 36 W 148 O 524 ] 76− I, [As III 5 Ce III 4 (H 2 O) 4 W 39 O 143 ] 25− II, [As III 3 (U VI O 2 ) 3 (H 2 O) 6 W 30 O 105 ] 15− III, [(W 5 O 18 ) 2 Ce III ] 9− IV, and [(W 5 O 18 ) 2 Th 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 WO 3 , x =0.066–0.162 with lattice parameters ranging from a =3.820(3) to 3.836(3) A, of thorium(IV) Th x WO 3 , x ∼0.1 with a =3.828(1) A, and of uranium(IV) U x WO 3 , x ∼0.1 with a =3.797(1) A 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.