Brown millerite

Fig.2. Structures of (a) Ca2Fe2Os (brown-millerite) and (b) Ca2Mn2Os. Oxygen vacancy ordering in the a-b plane is also shown.

In Tofield et al. s model for SrFe02.75, half of the iron atoms remain in octahedral coordination and half are in five-fold coordination. In view of the observed preference of Fe3+ for tetrahedral and octahedral coordination, and of Mn3+ for pyramidal coordination, we would expect that this proposed structure type would be more appropriate for manganate O2.75 perovskites than for those containing iron. Indeed, Grenier et al.(20) have reported a ferrite with the same oxygen stoichiometry, Ca4Fe2Ti20n, which has a structure related to that of brown-millerite, but with a 1 3 ratio of tetrahedral to octahedral layers. 

Lao.sSrijGao.eFei.aOs+s [24] and SrFeCoo.sOx [65] were also successfully used as MIECM for POM more than 1,000 h. A membrane reactor based on a brown-millerite structure materials could be continuously operated for over one year under syngas atmosphere at 900°C [66]. The syngas production rate was 60ml/cm. min, and equivalent oxygen permeation flux was 10-12 ml/cm. min. The composition of the membrane was not specified in the literature. 

This feature is frequently observed in brownmillerites and related phases (Sections 2.4.2, 2.4.3 and 2.4.5). Non-stoichiometric SrCoO can lose oxygen readily. In the range SrCoOj j rCoOjj, it appears to normal X-ray diffraction to be cubic, but electron microscopy shows it to consist of microdomains of brown-millerite-type material. 

Chemical expansion may be related to gradual changes in point defect concentrations, with emphasis on oxygen ion vacancies, and/or increase in cationic radii upon gradual reduction to a lower valence state. This is quite different from first-order redox reactions (e.g. Ni/YSZ cermet anodes) or phase transformations (e.g. brown-millerite to perovskite transformation Sr(Fe,Co)02.s - Sr(Fe,Co)03 3), with discontinuous structural changes and more or less abrupt property changes [65-67], except possibly for strong kinetic limitations [68] or 2-phase intermediate conditions... 

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