Brownmillerite-Related Phases

Electron microscopy has shown that under normal preparation conditions, rapidly cooled samples are rarely monophasic but contain intergrowths between the n=2,3, 4 and higher members of the series. Once again, preparations made under high oxygen pressures result in the filling of oxygen vacancies which is balanced by a variation in the valence of the Fe and Mn cations. 

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Ca2Fe205 and oxides of the CaMn03 v family are good examples of such vacancy-ordered structures. Complex intergrowth phases (ordered as well as disordered) involving brownmillerite and other related phases are commonly found in some of the anion-deficient oxides. Some of them also show polytypism due to different modes of stacking of the hexagonal and cubic layers. 

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. 

The brownmillerite structure type can be described as a sequence. .. OTOT... where O stands for the octahedral sheet and T the tetrahedral sheet. A number of other phases closely related to this structure have been characterized, including Ca2LaFe308, with a stacking sequence. .. OOTOOT... and Ca4Ti2Fe20i 1 and... 

In many of the AB03, j perovskites, nonstoichiometry spans the composition range 0 cubic structure when 0.0tetragonally distorted perovskite structure when 0.15brownmillerite phases when 0.28[Pg.46]

One of the first assumptions in our economic considerations was that all the technological challenges were met. This remains, at the moment of writing, uncertain. In the above discussion, a number of issues have passed, such as sealing technology, thermodynamic phase stability of the cubic perovskite versus brownmillerite, and kinetic demixing. In relation to this, we have not mentioned creep resistance as yet. For SCF, this was shown to be very low [40], which is likely to be the case for BSCF as well. 

The perovskite stracture tolerates relatively large nonstoichiometries and substitutions, while a number of perovskite-related stmctures (with perovskite building blocks) are formed when excesses, deficiencies or substitutions beyond the solubility limits lead to ordered defects and new phases. These comprise, for instance, oxygen deficiency (brownmillerite, ABO2.5), and A excess (K2NiF4-type structure, A2BO4) and ordered intermediate phases. 

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