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

The structure and ionic conductivity of fluorite-related Bi20Ca7NbO39.5, Bi10.75Ca4.375GaO22 and the high temperature form of Bi9ReO17, formed by quenching from 800 °C, were studied by neutron powder diffraction, X-ray powder diffraction and impedance spectroscopy. All materials formed distorted δ-Bi2O3–related monoclinic superstructures of a fluorite-related hexagonal subcell. The supercell, with P21/m symmetry, is derived from the cubic fluorite subcell axes (af, bf, cf) using the transformation matrix: − 1 1 1 0.5 0.5 0 1 − 1 1 . Both Bi20Ca7NbO39.5 and Bi10.75Ca4.375GaO22 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 Bi9ReO17 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.