The thermodynamic properties as well as oxygen exchange kinetics were examined on mixed ionic and electronic conducting ͑La 0.6 Sr 0.4 ͒ 0.99 FeO3 −␦ ͑LSF64͒ thin films deposited on MgO single crystals. It is found that thin films and bulk material have the same oxygen stoichiometry for a given temperature and oxygen partial pressure ͓i.e., the incorporation reaction has the same reaction enthalpy ͑⌬H 0 = −105 KJ/mol͒ and entropy ͑⌬S 0 = −75.5 J/mol/K͒ as found for bulk material͔. The thin film shows smaller apparent electrical conductivity than reported for bulk. This is due to imperfections in the film, which is not totally dense and contains closed porosity. Electrical conductivity relaxation was used to determine the surface exchange coefficient and its dependence on the temperature and oxygen partial pressure. Relaxation curves showed a good fit to a simple exponential decay. The vacancy surface exchange coefficient ͑k V ͒ determined from K chem shows a slope ͑log k V vs log P O 2 ͒ between 0.51 and 0.85. It is further found that k V is proportional to the product of the oxygen partial pressure and the vacancy concentration ͑k V ϰ P O 2 ␦ ϱ ͒. Different reaction mechanisms that can account for the observed P O 2 and ␦-dependence of k V are analyzed. It is proposed that the vacancies are the active sites of adsorption of molecular oxygen and that the rate determining step for the exchange reaction is splitting of the adsorbed oxygen. © 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3062941͔ All rights reserved.Manuscript submitted January 4, 2008; revised manuscript received October 8, 2008. Published February 12, 2009 Mixed ionic and electronic conductors ͑MIECs͒ have potential applications in solid oxide fuel cells ͑SOFCs͒, oxygen separation membranes, and thin-film sensors. One group of promising materials with high oxygen-ion conductivity, good catalytic activity for oxygen reduction, and high chemical stability in reducing atmosphere is strontium-doped lanthanum ferrite ͑LSF͒ or La 1−x Sr x FeO 3 . Presently, the search for superior SOFC cathode or oxygen membrane material is to a large degree based on a trial-and-error approach. An understanding of the detailed mechanism of the oxygen reduction process over model cathode ͑or membrane͒ materials, such as LSF, would enable researchers to make more qualified guesses on what materials to choose and how to optimize these by, e.g., partial substitution of the elements. In this study, different reaction mechanisms for the oxygen reduction on LSF are described and compared to experimental investigations carried out on a thin film.The oxygen transport in a MIEC is determined by the oxygen exchange over the gas-solid interface and diffusion of both oxide ions and electrons/holes in the bulk. The oxygen transport is generally limited by the surface exchange process or by the bulk diffusion, depending on membrane thickness because the electronic charge carrier density and mobility is large when compared to those of the oxide ion. The ratio of the oxygen chemica...

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