Phase brownmillerite

Oxygen bacancies are commonly encountered in oxide perovskites. In AB03 unlike in W03, and Ti02 crystallographic shear planes are not found. Instead, a variety of superstructures are seen due to the ordering of vacancies. The brownmillerite phase of... 

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

Brownmillerite is a mineral of approximate composition CajFeAlOj. Mineral specimens vary in real composition with isomorphous substitution of both the Fe and A1 components being commonplace. A large number of brownmillerite phases are now known, the simplest having a formula A B Oj, typified by BajIn Oj, but many others, involving partial substitution of the A cations to form A2 A B20j, the B cations to form A B Bp (as in brownmillerite itself) or both A and B cations, have been synthesised (Table 2.4). 

Brownmillerite phases form an important constituent of cement powder. Portland cement, one of the most widely used types, is made by heating mixed limestone and clay in a kiln. This results in multiple reactions together with partial melting and sintering of the mixture, producing a dense mass called clinker, which is ground to produce cement powder. The resulting complex material contains four major... 

The real stmcture of brownmillerite phases is more difficult to resolve because the apex-linked chains of tetrahedra are not regular (Figure 2.8a) but display a cormgation compared to the ideal motif. There are two possibilities for this chain deformation, either right (R) or left (L) handed (Figure 2.8b and c). The organisation of these chains has repercussions for the space group and unit cell axes of the brownmillerite phase ... 

The thermal history of the sample may be important. The orthorhombic brownmillerite phase StjCOjOj (which has also been reported to be Co deficient, as SrgCOjOjj) is only obtained when samples are heated in air and quenched rapidly to room temperature from above 900°C. If this quenched material is heated, it transforms at 653°C to the hexagonal BaNiOj structure (Section 3.1) and then at between 920 and 940°C to a disordered cubic phase space group Pm3m, a=0.395286miL... 

In the well-studied system Ba2lu2. Ti Oj, which can be considered to form within the system Ba2ln205 brownmillerite and BaTi03 (Ba2Ti20g) perovskite, the brownmillerite phase persists only up to 0 composition limits of 0.075

When B-site ions can take on a variety of oxidation states, a change in the external oxygen partial pressure can lead to an oxygen composition range for the brownmillerite phase. This additional oxygen has a number of structural consequences. The oxygen can occupy the vacancies in the brownmillerite phase and lead to new ordered phases, new coordination polyhedra such as trigonal prisms or square pyramids, discussed below (Section 2.5), or result in a disordered cubic perovskite phase. 

An extensively studied series of brownmillerite phases can be prepared from the cubic perovskite series La, A MnOj, where A is Ca, Sr or Ba. The oxidised cubic phases contain Mn " and the ratio dependent upon the value of x. The cubic form can be reduced to browmnillerite structures La, A MnO., where all the B-site cations are now Mn " and Mn. The approximate phase ranges, x, over which the browmnillerite structure exists are as follows Ca, 0.225-0.5 Sr, 0.155-0.5 and Ba, 0.11-0.325. 

Figure 2.11 Simplified phase diagram for the LaFeO (perovskite)-CaFeO (Ca FeJD brownmillerite) phase region. The perovskite phase has a symmetry change from orthorhombic Pnma at lower temperatures to trigonal Rlc at higher temperatures...

The Sr-Co-Fe perovskite systems have the highest oxygen permeabilities of all the perovskite MIECs but can suffer from degradation as a result of the formation of a brownmillerite phase. Experimental and modeling work concerning the transport... 

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