Competitive oxide ion conductivity has been identified recently in members of the Ba 3 Nb 1-y (Mo 1-2) series, which adopt a disordered rhombohedral "hybrid" structure combining features of the 9R perovskite and palmierite structures. We report the first growth of Ba 3 NbMoO 8.5 and Ba 3 NbWO 8.5 single crystals from molten phases and their characterisation using single-crystal x-ray diffraction data between 120 and 473 K. Structure refinements reveal a previously unreported splitting of the central Nb/M cation site, rationalised by bonding considerations, which imposes limitations on the material stoichiometry and possible arrangements of cations within the face-sharing polyhedral stacks. Analysis of atomic displacement parameters and bond valence energy landscapes (BVELs) gives new insight into the probable low-energy pathways for oxide ion diffusion in the hybrid structure, indicating that they are three-dimensional and involve all crystallographically distinct oxygen sites. Evidence for considerable static disorder of the oxide ions at temperatures below the onset of significant conductivity is also discussed.

We report a detailed study of the dynamics of oxide ionic conduction in brownmillerite-type Sr 2 Fe 2 O 5 , including lattice anisotropy, based on neutron scattering studies of a large (partially twinned) single crystal in combination with ab initio molecular dynamics simulations. Single-crystal diffraction reveals supercell peaks due to long-range ordering among chains of corner-sharing FeO 4 tetrahedra, which disappears on heating above 540 °C due to confined local rotations of tetrahedra. Our simulations show that these rotations are essentially isotropic, but are a precondition for the anisotropic motion that moves oxide ions into the tetrahedral layers from the octahedral layers, which we observe experimentally as a Lorentzian broadening of the quasielastic neutron scattering spectrum. This continual but incoherent movement of oxide ions in turn creates conduction pathways and activates long-range diffusion at the interface between layers, which appears to be largely isotropic in two dimensions, in contrast with previously proposed mechanisms that suggest diffusion occurs preferentially along the c axis.

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Ba3NbMoO8.5 has recently been demonstrated to exhibit competitive oxide ion conductivity and to be stable under reducing conditions, making it an excellent potential electrolyte for solid oxide fuel cells.