The defect structure and electrical properties of the fast oxide ion-conducting solid electrolyte δ-Bi 3 YO 6 have been studied using a combination of total neutron scattering analysis, energy minimization methods, and AC impedance spectroscopy. Conventional structural analysis using the Rietveld method reveals the oxide ions to be distributed over three crystallographic sites at room temperature, with a small change in this distribution at 800 °C. Analysis of short-range correlations using a total neutron scattering approach yields information on Bi and Y coordination environments. Careful analysis of the angular distribution functions derived from reverse Monte Carlo modeling of the total scattering data reveals physical evidence for a predominance of AE110ae vacancy ordering in this system. This ordering is confirmed as the lowest energy configuration in parallel energy minimization simulations.

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Reverse Monte Carlo (RMC) modelling of neutron total scattering data, combined with conventional Rietveld analysis of x-ray and neutron data, has been used to describe the cation coordination environments and vacancy pair distribution in the oxide ion conducting electrolyte Bi3YbO6. The thermal variation of the cubic fluorite unit cell volume, monitored by variable temperature x-ray and neutron experiments, reveals significant curvature, which is explained by changes in the oxide ion distribution. There is a significant increase in tetrahedral oxide ion vacancy concentration relative to δ-Bi2O3, due to the creation of Frenkel defects associated with the Yb(3+) cation. The tetrahedral oxide ion vacancy concentration increases from room temperature to 800 °C, but little change is observed in the vacancy pair distribution with temperature. The vacancy pair distributions at both temperatures are consistent with a favouring of [100] vacancy pairs.

The di-substituted bismuth oxide based oxide ion conducting electrolyte system Bi 2.5+x Pb 0.5 YO 5.75+3x/2-δ (x = 0, 1 and 2) has been investigated by X-ray powder diffraction, a.c. impedance spectroscopy, thermal analysis and X-ray photoelectron spectroscopy. δ-Bi 2 O 3 type phases are observed for all compositions studied and showed no obvious changes in structure up to 850°C. Results from thermal analysis, and the thermal variation of the lattice parameter indicate a small degree of reduction occurs at high temperature, which is preserved on quenching. XPS results suggest this involves reduction of bismuth and lead cations. This reduction is reversed on heating at intermediate temperatures. The redox reactions appear to be correlated to a degree of curvature in the Arrhenius plot of conductivity. Measurement of transference numbers indicate that these materials are predominantly ionic conductors, with conductivity values at high temperature comparable with mono-substituted analogues.