Oxygen vacancy formation and ionic transport in Sr4Fe6O13±δ

Oxygen permeation through dense Sr4Fe6O13+/-delta membranes was found to be limited by the bulk ambipolar conductivity. At 1173 K and oxygen partial pressures 10 to 1.01x10(5) Pa, ionic conductivity of intergrowth strontium ferrite increases with reducing oxygen pressure, indicating that the vacancy migration mechanism provides a greater contribution to ionic transport in comparison with the interstitial diffusion. The ion transference numbers of Sr4Fe6O13+/-delta increase from 2.5x10(-4) to 1.9x10(-3) when the oxygen pressure decreases. Combined X-ray and neutron powder diffraction studies of oxygen-deficient Sr4Fe6O13-delta showed an accumulation of oxygen vacancies in the non-perovskite layers, built of oxygen-iron polyhedra with pentacoordinated Fe cations. Both thermal and chemically-induced expansion of Sr4Fe6O13+/-delta lattice have pronounced anisotropic character and are considerably lower than that typical for perovskite-type strontium ferrite. The average thermal expansion coefficients of Sr4Fe6O13+/-delta ceramics at 770-1100 K in oxidizing and reducing atmospheres vary in the range (10.8-13.2)x 10(-6) K-1.