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

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 ((sic) = 850 degrees C) at appropriate oxygen pressures. These new phosphates show substitution of W5+ by either M1/33+W2/36+ (M: V, Cr, Fe, Mo) or Ti1/24+W1/26+. Members of the MPTB structural series with m = 2 [e.g. (Cr4/3W8/3O12)-W-III-O-VI(PO2)(4); (Ti6/3W6/3O12)-W-IV-O-VI(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 WOPO (4) (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 (P2(1)/m, Z = 1, 80 parameters, 1832 independent reflections R-1 = 0.027, wR(2) = 0.063). The crystal structures of (Mo4/3W8/3O12)-W-III-O-VI(PO2)(4) and (Cr4/3W8/3O12)-W-III-O-VI(PO2)(4) (MPTBs with m = 2) were also refined from single-crystal data {(Mo/W (Cr/W): P2(1)/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 W-VI over the metal sites. By selected area electron diffraction the unit cell dimensions of (Cr4/3W8/3O12)-W-III-O-VI(PO2)(4) and (Cr4/3W20/3O24)-W-III-O-VI(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.