Re-investigation of the structure and crystal chemistry of the Bi2O3-W2O6 'type (Ib)' solid solution using single-crystal neutron and synchrotron X-ray diffraction.

Single crystals of composition Bi(35.66)W(4.34)O(66.51) (or Bi(8.2)WO(15.3), bismuth tungsten oxide), within the type (Ib) solid-solution region of the Bi(2)O(3)-WO(3) system, were synthesized using the floating-zone furnace method. Synchrotron X-ray and neutron single-crystal diffraction data were used to confirm the previously tentative assignment of the room-temperature space group as I4(1). Fourier analysis of the combined X-ray and neutron datasets was used to elucidate and refine fully the cation and anion arrays for the first time. The mixed cation site M1 is shown to be coordinated by eight O atoms in an irregular cube when M = Bi, and by six O atoms in an octahedron when M = W. The resulting disorder in the average structure around M1 is discussed in the context of experimentally observed oxide-ion conductivity.