The O permeability of CaAl0.5Fe0.5O2.5+δ brownmillerite membranes at 1123 to 1273K was found to be limited by the bulk ionic conduction, with an activation energy of 170kJ/mol. The ion transference numbers in air were in the range 2 x 10-3 to 5 x 10-3. The analysis of structural parameters showed that the ionic transport in the CaAl0.5Fe0.5O2.5+δ lattice was essentially along the c axis. The largest ion-migration channels were found in the perovskite-type layers formed by Fe–O octahedra, though diffusion in tetrahedral layers of the brownmillerite structure was also possible. Heating up to 700–800K in air led to losses of hyperstoichiometric O, accompanied with a drastic expansion and, probably, partial disordering of the CaAl0.5Fe0.5O2.5+δ lattice. The average thermal expansion coefficients of CaAl0.5Fe0.5O2.5+δ ceramics in air were 1.67 x 10-5 and 1.26 x 10-5/K at 370 to 850 and 930 to 1300K, respectively.

Oxygen Ionic Conduction in Brownmillerite CaAl0.5Fe0.5O2.5+ δ. V.V.Kharton, I.P.Marozau, N.P.Vyshatko, A.L.Shaula, A.P.Viskup, E.N.Naumovich, F.M.B.Marques: Materials Research Bulletin, 2003, 38[5], 773-82