The structure and ionic conductivity of the fluorite-related isostructural materials Bi_(20)Ca_7NbO_(39.5), Bi _(10.75)Ca_(4.375)GaO_(22) and quenched Bi _9ReO_(17)

摘要

The structure and ionic conductivity of fluorite-related Bi _(20)Ca_7NbO_(39.5), Bi_(10.75)Ca _(4.375)GaO_(22) and the high temperature form of Bi _9ReO_(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_2O _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: -1110.50.501-11. Both Bi_(20)Ca _7NbO_(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_9ReO _(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.