Abstract
Oxygen permeation through dense Sr4Fe6O13±δ membranes was found to be limited by the bulk ambipolar conductivity. At 1173 K and oxygen partial pressures 10 to 1.01×105 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±δ increase from 2.5×10–4 to 1.9×10–3 when the oxygen pressure decreases. Combined X-ray and neutron powder diffraction studies of oxygen-deficient Sr4Fe6O13–δ 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±δ lattice have pronounced anisotropic character and are considerably lower than that typical for perovskite-type strontium ferrite. The average thermal expansion coefficients of Sr4Fe6O13±δ ceramics at 770–1100 K in oxidizing and reducing atmospheres vary in the range (10.8–13.2)×10–6 K–1.
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