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 similar to 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 L-III- and Re L-III- 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 x 10(-5) Omega(-1) cm(-1) at 673 K, whereas the disordered form shows higher oxide ion conductivity (9.1 x 10(-4) Omega(-1) cm(-1) at 673 K). (C) 2010 Elsevier Inc. All rights reserved.