The structure and ionic conductivity of the fluorite-related isostructural materials Bi20Ca7NbO39.5, Bi10.75Ca4.375GaO22 and quenched Bi9ReO17

Abstract The structure and ionic conductivity of fluorite-related Bi 20 Ca 7 NbO 39.5 , Bi 10.75 Ca 4.375 GaO 22 and the high temperature form of Bi 9 ReO 17 , formed by quenching from 800 °C, were studied by neutron powder diffraction, X-ray powder diffraction and impedance spectroscopy. All materials formed distorted δ-Bi 2 O 3 –related monoclinic superstructures of a fluorite-related hexagonal subcell. The supercell, with P2 1 / m symmetry, is derived from the cubic fluorite subcell axes ( a f , b f , c f ) using the transformation matrix: − 1 1 1 0.5 0.5 0 1 − 1 1 . Both Bi 20 Ca 7 NbO 39.5 and Bi 10.75 Ca 4.375 GaO 22 are shown to display good oxide ion conductivity (2.52 × 10 − 5  Ω − 1  cm − 1 and 1.02 × 10 − 5  Ω − 1  cm − 1 at 673 K with activation energies of 1.12 eV and 1.25 eV, respectively); quenched Bi 9 ReO 17 has enhanced oxide ion conductivity (1.44 × 10 − 3  Ω − 1  cm − 1 at 673 K) with a lower activation energy of 0.76 eV.