Brownmillerite structures brownmillerites

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. 

MnO,.5..., the structure of Ca2Mn2Os is obtained. An important feature of the structure which distinguishes it from brownmillerite is that manganese is square-pyramidally coordinated. It has been... 

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

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. 

Consider the brownmillerite (A2B2O5) stmcture examined earlier. This structure consists of alternating layers of vertex-sharing BO3 octahedra and BO2 tetrahedra. It is immediately obvious to the synthetic strategist that one has the opportunity to place cations with specific coordination preferences exclusively in one type of layer to give A2 B0 B 02), as was previously discussed. For example, in Ca2MnGaOs, Mn " exclusively occupies the BO3 layers. In this case, that would have been predicted by a consideration of the CFSE. 

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