Substitution of W5+ in monophosphate tungsten bronzes by combinations M n+/W6+

Abstract

Abstract A series of hitherto unknown mixed-metal phosphates of the monophosphate tungsten bronze structure family [MPTB; (WO3)2m (PO2)4] have been obtained by solution combustion synthesis followed by annealing (ϑ = 850°C) at appropriate oxygen pressures. These new phosphates show substitution of W5+ by either M 3+ 1/3W6+ 2/3 (M: V, Cr, Fe, Mo) or Ti4+ 1/2W6+ 1/2. Members of the MPTB structural series with m = 2 [e.g. CrIII 4/3WVI 8/3O12(PO2)4; TiIV 6/3WVI 6/3O12(PO2)4] and m = 4 [e.g. Cr4/3W20/3O24(PO2)4] have been obtained. In the course of our investigation the crystal structure of WOPO4 (MPTB with m = 2: W4O12(PO2)4) has been re-determined from X-ray single-crystal data, showing monoclinic instead of the orthorhombic symmetry reported in literature (P21 /m, Z = 1, 80 parameters, 1832 independent reflections R 1 = 0.027, wR 2 = 0.063). The crystal structures of MoIII 4/3WVI 8/3O12(PO2)4 and CrIII 4/3WVI 8/3O12(PO2)4 (MPTBs with m = 2) were also refined from single-crystal data {(Mo/W (Cr/W): P21 /m, Z = 1, 80(86) parameters, 1782(1769) independent reflections, R 1 = 0.035(0.059), wR 2 = 0.081(0.146)}. These refinements indicate statistical distribution of M III and WVI over the metal sites. By selected area electron diffraction the unit cell dimensions of CrIII 4/3WVI 8/3O12(PO2)4 and CrIII 4/3WVI 20/3O24(PO2)4 derived from XRPD and SXRD are confirmed. HRTEM images of Cr4/3W20/3O24(PO2)4 are in agreement with its assumed close structural relation to W8O24(PO2)4 and show an highly ordered atomic arrangement.