The oxygen permeation properties of barium bismuth iron oxide were investigated within the family of [Ba2−3xBi3x−1][Fe2xBi1−2x]O2+3x/2 for x = 0.17 to 0.60. The structure changed progressively from cubic to tetragonal and then to hexagonal as function of x in accordance with the different relative amounts of bismuth on A-site and B-site of ABO3−δ perovskite lattices. It was found that the oxygen flux and electrical conductivity correlated strongly, and it was prevalent for the cubic structure (x = 0.33 to 0.40) which conferred the highest oxygen flux of 0.59ml/min.cm2 at 950C for a disk membrane x = 0.33 with a thickness of 1.2mm. By reducing the thickness of the disk membrane to 0.8 mm, the oxygen flux increased to 0.77ml/min.cm2, suggesting both surface kinetics and ion diffusion controlled oxygen flux, though the former was more prominent at higher temperatures. For disk membranes x = 0.45–0.60, the perovskite structure changed to tetragonal and hexagonal, and the oxygen flux was insignificant below 900C, clearly indicating electron conduction properties only. However, for two compositions with relatively high bismuth content, e.g. x = 0.55 and 0.60, there was a sudden and significant rise of oxygen permeability above 900C, by more than one order of magnitude. These materials changed conduction behavior from metallic to semiconductor at around 900C. These results suggested the advent of mixed ionic electronic conducting properties caused by the structure transition as bismuth ions changed their valence states to compensate for the oxygen vacancies formed within the perovskite lattices.

Structure Effect on the Oxygen Permeation Properties of Barium Bismuth Iron Oxide Membranes. J.Sunarso, S.Liu, J.C.Diniz da Costa: Journal of Membrane Science, 2010, 351[1-2], 44-9