Brownmillerite perovskite-type structure

The oxide Ba2In205 is another well-studied phase that adopts the brownmillerite structure. This material disorders above 930°C to a perovskite-type structure containing oxygen vacancies. Both the Sr-Fe and Ba-In oxides are of interest for electrochemical applications in fuel cells and similar devices (Section 6.10). 

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. 

Oxides with close-packed oxygen lattices and only partially filled tetrahedral and octahedral sites may also facilitate diffusion of metal ions in the unoccupied, interstitial positions. Finally, even large anions may diffuse interstitially if the anion sublattice contains structurally empty sites in lines or planes which may serve as pathways for interstitial defects. Examples are rare earth sesquioxides (e.g. Y2O3) and pyrochlore-type oxides (e.g. La2Zr207) with fluorite-derived structures and brownmillerite-type oxides (e.g. Ca2Fe205) with perovskite-derived structure. 

Perovskite-like cuprates with the brownmillerite-type structure RACuGaOs (R = La, Pr, Nd A = Ca, Sr) (Gal201) [152, 153] provide a structure suitable for superconductivity. The orthorhombic brownmillerite structure am4ap, b m c m Op /l) is represented as a sequence of layers (Figure 13.20) ... 

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]

Ba2ln20s-Based Oxides The brownmillerite-type oxide has the general formula A2B2O5, with the crystal structure presented in Fig. 6.6. Alkaline-earth metal ions such as Ba occupy the A site, and trivalent typical metal ions such as In occupy the B site [92-99]. At temperatures below 1,140 K, it behaves as a mixed conductor which shows both p-type electronic conductivity and oxygen ionic conductivity due to its oxygen excess [92]. When it is heated to 1,140-1,230 K, ionic conductivity increases, which is associated with the phase transition to the disordered perovskite structure, as shown in Fig. 6.7 [93]. The ionic transference number also rises drastically to unity. 

Oxide ion conductors with the perovskite structure mentioned above belong to so-called single perovskites, which can be expressed as the simple form, ABO3. Besides these, there are different types of perovskite-related oxides and some of them are known to show oxide ion conduction. One of them is Brownmillerite, Ba2ln205. This composition can be written as Baln02.5, and... 

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