DEFECTS AND TRANSPORT OF THE BROWNMILLERITE OXIDES WITH HIGH OXYGEN-ION CONDUCTIVITY - BA2IN2O5

A defect chemistry model is proposed for the brownmillerite oxides wit h high oxygen ion conductivity. Ba2In2O5 was chosen as the model mater ial and its electrical conductivity and transport properties have been studied in detail. The oxygen ion conduction above the order-disorder temperature, T-d approximate to 925 degrees C, and the mixed ionic-el ectronic conduction below T-d, was studied by conductivity and EMF mea surements as a function of temperature and oxygen activity. The main d efects are intrinsic anion Frenkel defects below T-d, and above T-d th e oxide can be treated as acceptor-doped perovskite with extrinsic oxy gen vacancies. Charge compensation involves only ionic defects over th e whole P-O2 range used in this study. The formation and mobility enth alpies of the Frenkel defects, the redox enthalpies, and the band gap have been obtained for this oxide. The proposed model is in good agree ment with the experimental results. Decomposition of Ba2In2O5 with red uction of In3+ is indicated by the conductivity measurements. Degradat ion in CO2 atmospheres with formation of BaCO3 and volatilization of I n2O3 was also observed by XRD and EDS techniques. It is clear that In3 + will have to be replaced by an element with a more stable 3 + oxidat ion state (e.g. Y, Er, Ga), if a brownmillerite compound is to be a us eful solid electrolyte over a wide range of oxygen activities.