The ionic conductivity and local environment of cations in Bi9ReO17

The influence of temperature on the structure of Bi9ReO17 has been investigated using differential thermal analysis, variable temperature X-ray diffraction and neutron powder diffraction. The material undergoes an order–disorder transition at ∼1000 K on heating, to form a fluorite-related phase. The local environments of the cations in fully ordered Bi9ReO17 have been investigated by Bi LIII- and Re LIII-edge extended X-ray absorption fine structure (EXAFS) measurements to complement the neutron powder diffraction information. Whereas rhenium displays regular tetrahedral coordination, all bismuth sites show coordination geometries which reflect the importance of a stereochemically active lone pair of electrons. Because of the wide range of Bi–O distances, EXAFS data are similar to those observed for disordered structures, and are dominated by the shorter Bi–O bonds. Ionic conductivity measurements indicate that ordered Bi9ReO17 exhibits reasonably high oxide ion conductivity, corresponding to 2.9×10−5 Ω−1 cm-1 at 673 K, whereas the disordered form shows higher oxide ion conductivity (9.1×10−4 Ω−1 cm−1 at 673 K).