Oxide brownmillerite-type

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. 

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. 

Figure 6.19 shows the temperature dependence of the 0 -ion conductivity in an oxygen-deficient brownmillerite-type perovskite Ba2ln20s that exhibits a first-order order-disorder transition at 930 °C. The Arrhenius plot of 0 -ion conductivity shows that, above T, a considerable short-range order persists nevertheless, the oxide-ion conduction in this field can be competitive with that in the commercial electrolyte yttria-stabilized zirconia (YSZ). 

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). 

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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