Combined Experimental and Computational Study of Oxide Ion Conduction Dynamics in Sr2Fe2O5 Brownmillerite

We report a detailed study of the dynamics of oxide ionic conduction in brownmillerite-type Sr2Fe2O5, induding 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 FeO4 tetrahedra, which disappears on heating above 540 degrees 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.