Oxygen ionic conduction in brownmillerite CaAl0.5Fe0.5O2.5+

Abstract The oxygen permeability of CaAl 0.5 Fe 0.5 O 2.5+ δ brownmillerite membranes at 1123–1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2×10 −3 to 5×10 −3 . The analysis of structural parameters showed that the ionic transport in the CaAl 0.5 Fe 0.5 O 2.5+ δ lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron–oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700–800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl 0.5 Fe 0.5 O 2.5+ δ lattice. The average thermal expansion coefficients of CaAl 0.5 Fe 0.5 O 2.5+ δ ceramics in air are 16.7×10 −6 and 12.6×10 −6 K −1 at 370–850 and 930–1300 K, respectively.