Brownmillerite structures

Seeking for solutions to lower Tf in Ba2ln20s, Goodenough et al. [211,212] extended the foresaid investigations to include Ba3bi2M08 (M = Zr, Hf, Ce). Ideal ordering of the oxygen vacancies in these compounds would render them 

---

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... 

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 oxygen composition of doped brownmillerite structure materials is not always exactly 5.0 and depends upon a number of factors, especially the surrounding oxygen partial pressure. Thus, the oxide formed when Fe is replaced by Co has a broad... 

Figure 4.34 Brownmillerite structure (a) ideal perovskite structure (circles indicate oxygen atoms that need to be removed to convert octahedra to tetrahedra in brownmillerite) and (b) idealized brownmillerite structure consisting of sheets of octahedra and tetrahedra.

Compounds with the A2B2C>5 brownmillerite structure disorder at high temperatures to form a structure of the ... 

In the brownmillerite structure, the oxygen vacancies order in such a way that half the iron of CaFe02.5 are octahedrally coordinated and half are tetrahedrally coordinated. This ordering is an expression of the tetrahedral-site stability of Fe ions. In a system like Lai yCayFe03 x < y/2, there is a tendency to form intergrowths of perovskite and brownmillerite structures the oxygen vacancies do not remain randomly distributed. Mossbauer spectra at 4.2 K for this complex system exhibit three sextets they have been interpreted in terms of 2 Fe and one Fe " ", or of an Fe and the disproportiona-... 

One of the best-characterized perovskite oxides with ordering of anion vacancies is the brownmillerite stmctme exhibited by Ca2Fe205 and Ca2FeA105 (Grenier et al, 1981). The compositions could be considered as anion-deficient perovskites with one-sixth of anion sites being vacant. The orthorhombic unit cell of the brownmillerite structure (a = 5.425, b = 5.598 and c = 14.768 A for Ca2Fe205) arises because of vacancy-ordering and is related to the cubic perovskite as a - c-... 

Figure 5.25 (a) Vacancy ordering in the ab plane of CajFejOj of brownmillerite structure (b) alternating sequence of octahedral and tetrahedral layers in the c direction. Small filled circles, iron large filled circles, oxygen open squares, oxygen vacancy. 

Plate 6—Lattice image of Ca,La,, FeOi, showing disordered intergrowth. B stands for brownmillerite structure and G stands for... 

Figure 50 Idealized representation of the octahedral and tetrahedral networks which make up the skeletons of (a) the perovskite structure, (b) the brownmillerite structure A2M2O5 (n = 2), (c) the AilVfOa structure (n = 3), and (d) the A M Oi 1 structure (n = 4) (Reproduced by permission from Mater. Res. Bull., 1978, 13, 329)...

A point defect model has been proposed for the brownmillerite structure, based on a study of electrical conductivity and em/measurements of Ba2ln205 [236]. The experimental results matched well with modelling, while evidence was found for protonic conduction at low temperatures. In modelling, the unoccupied oxygen sites relative to the perovskite structure, below Tj, are regarded as structural units and therefore are potential interstitial sites for... 

Figure 3.7 Brownmillerite structure as an example of long-range oxygen vacancy ordering. O — octahedral formed by oxygen (open circles) and B cations (closed circles). T — tetrahedra involving B-site cations. The squares denote oxygen vacancies. For clarity, the A-site cations are not shown.

In the follwing we propose an original structural model for the AM03 y phases based on the relationship between the perovskite and brownmillerite structure types45. ... 

Several oxides with perovskite related structures can also be intercalated with oxygen ions by an electrochemical method. The oxide Sr2Fe20s with the brownmillerite structure has been electrochemically oxidized to SrFeOa. The reaction was carried out by controlled potential electrolysis at a potential below that for oxygen evolution in 1 M aqueous KOH at room temperature. Bulk oxidation was confirmed by Mossbauer spectroscopy and X-ray diffraction. Similar results have been obtained for electrochemical oxidation... 

CaO —FeO... A slight shift of the iron atoms in the FeO layers leads to a tetrahedral coordination of iron in this layer resulting in alternate sheets of FeOg octahedra and FeOa tetrahedra (OTOT. ) in the c-direction (Fig. 6b). We have recently prepared a new oxide, Ca2FeCoOs, possessing the brownmillerite structure wherein Fe and Co occupy tetrahedral and octahedral sites respectively. 

---