A defect chemistry model was proposed for brownmillerite oxides with a high O conductivity. The present material was used as a model, and its conductivity was studied. The O ion conductivity above the order-disorder temperature (925C), and a mixed ionic-electronic conduction at lower temperatures, were investigated by means of conductivity and electromotive force measurements. It was found that the main defects below 925C were intrinsic anion Frenkel defects. At higher temperatures, the oxide could be treated as an acceptor-doped perovskite with extrinsic O vacancies. Charge compensation involved only ionic defects over the entire O partial pressure range which was used here. The formation and mobility enthalpies of the Frenkel defects were deduced. The proposed model was in good agreement with the experimental results. Decomposition of the oxide, with a reduction in In3+, was revealed by the conductivity measurements. It was concluded that the In3+ would have to be replaced by an element (such as Y, Er or Ga) with a more stable 3+ oxidation state if brownmillerite compounds were to be useful solid electrolytes over a wide range of O activities.

G.B.Zhang, D.M.Smyth: Solid State Ionics, 1995, 82[3-4], 161-72