A combined experimental and computational study of oxide ion conduction dynamics in Sr2Fe2O5 brownmillerite

Abstract

We report a detailed study of the dynamics of oxide ionic conduction in brownmillerite-type Sr2Fe2O5, including lat-tice 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 order-ing among chains of corner-sharing FeO4 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 tet-rahedral layers from the octahedral layers, which we observe experimentally as a Lorentzian broadening of the quasielas-tic neutron scattering spectrum. This continual but incoher-ent 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 mecha-nisms that suggest diffusion occurs preferentially along the c axis.