Thermal variation of structure and electrical conductivity in Bi14WO24

Abstract: Structure and electrical conductivity of Bi14WO24 as a function of temperature have been examined by X-ray and neutron powder diffraction, a.c. impedance spectroscopy and differential thermal analysis. The room temperature structure was successfully refined using a monoclinic subcell model in space group I2/m. However, additional reflections in the neutron data are consistent with a large supercell of dimensions a =17.3780(1)Å, b =17.3891(1)Å, c =26.1785(2)Å and β=90.270(1)°, as previously proposed. Transitions to tetragonal and cubic phases are observed at ca. 35°C and 780°C, respectively. The structure of the high temperature polymorph is confirmed as a fully disordered δ-Bi2O3 type phase. Analysis of the defect structure is consistent with a predominantly tetrahedral environment for tungsten, as seen at low temperatures. The conductivity behaviour is correlated with the appearance of the δ-phase at high temperatures and exhibits a value of 0.97Scm−1 at 800°C. [Copyright &y& Elsevier]