Atomic Resolution Analysis of the Defect Chemistry and Microdomain Structure of Brownmillerite-Type Strontium Cobaltite Yasuo Ito,* ,† Robert F. Klie, and Nigel D. Browning* Department of Physics, University of Illinois, Chicago, Illinois 60607-7059 Terry J. Mazanec* BP Amoco Chemicals, Naperville, Illinois 60566-7011 To investigate the fundamental aspects of vacancy ordering in oxygen-transporting ceramic membranes, we have per- formed atomic resolution analysis of individual domains in brownmillerite-type SrCoO 3 . Electron energy loss spec- troscopy indicates that the Co valence state in adjacent planes can be 2 and 4. This charge localization is accom- panied by oxygen deficiency and the formation of ordered octahedral and tetrahedral coordinated Co sites. At microdo- main boundaries, Z-contrast images reveal a structural relax- ation of the octahedral site with the reduction of the Co valence state from 4 to 3 and the incorporation of extra oxygen vacancies. I. Introduction O XYGEN-DEFICIENT perovskite oxides are of interest for appli- cations such as electrodes in solid oxide fuel cells and potentially as oxygen separation membranes (for their high mo- bility of oxygen vacancies at elevated temperatures). 1–3 Many of the physical properties of transition-metal perovskite oxides (ABO 3 ), such as the ionic and electronic conductivity, are related to the mixed valence state of the B-site transition metal, and the presence of oxygen vacancies. Both can be controlled by the substitution of lower-valence cations (e.g., with a formal valence state of 2+) in place of a cation with a formal valence state of 3+ on the A-site, by the temperature, and by the oxygen partial pressure. The valence states of cations and the possible ordering of oxygen vacancies are therefore critical issues in the performance of devices. In many oxygen-deficient perovskite oxides vacancies become ordered under certain temperature and reduced atmo- sphere conditions, 4 and they transform into, for example, the brownmillerite structure (Fig. 1). At an even higher temperature, certain oxides with the brownmillerite-type structure transform further into the perovskite structure with randomly distributed oxygen vacancies, again depending on the oxygen partial pres- sure of the processing atmosphere. This order– disorder transi- tion corresponds with a dramatic increase in the oxygen permeability. 5,6 Complex perovskite-type oxygen-deficient oxides, for example SrCo 1-x B' x O 3- (B'= Cr, Fe, Co, and Cu) or La 1-x Sr x CoO 3- , are candidate materials for the electrodes in solid oxide fuel cells and oxygen separation membranes described above. To understand the detailed atomic-scale structure–property relationship of these oxides, analysis of the end members of the series, for example SrCoO 3- , LaCoO 3- , or SrFeO 3- , can provide fundamental insight into their structure/defect chemistry. In this paper we investigate one of these end members, SrCoO 3- , by employing a T. E. Mitchell—contributing editor Manuscript No. 187885. Received March 2, 2001; approved December 26, 2001. Supported by the U.S. Department of Energy under Grant No. DE-FC26- 99FT40054 and by the National Science Foundation under Grant No. NSF-DMR- 9601792. Additional support was provided by BP Chemicals. *Member, American Ceramic Society. † Currently with the Department of Physics, Northern Illinois University, DeKalb, Illinois 60115. Fig. 1. Three-dimensional schematic of the brownmillerite structure. Vacancy ordering in alternate (001) BO 2 planes along the [110] p direction of the cubic perovskite parent unit (dotted lines). 4 Atomic columns a, b, and c correspond to those in Figs. 3 and 5. J. Am. Ceram. Soc., 85 [4] 969 –76 (2002) 969 journal