Anion-deficient perovskites

Ordering of vacancies also plays a key role in selective oxidation catalysis over perovskite-based catalysts such as CaMnOs oxides. CaMnOs has a CaTiOs (AMO3) perovskite structure which is made up of cations coordinated to 12 0 anions. They, in turn, are connected to corner-sharing MoOe octahedra. CaMnOs was used as a model catalyst on a laboratory scale by Thomas et al (1982) in propene oxidation to benzene and 2-methyl propene to paraxylene. In such reactions the compounds are found to undergo reduction to form anion-deficient metastable phases of the type CaMnOs-x where 0 < x < 0.5, forming several distinct phases. 

Figure 6.1. (d) Perovskite transformations of LaNi03 thermogravimetriac analyses (TGA) data in air (broken curve) and in oxygen (b) ED of anion-deficient LaNiOs in [100], showing superstructure due to anion vacancy ordering arrowed. (After Gai and Rao Z. Naturforsch. a 30). 

Anion-deficient perovskites and vacancy-ordered structures. Anion-vacancy... 

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-... 

Anion-deficient nonstoichiometry in AB03 x perovskites is not accommodated by the CS mechanism. The reason probably is that the constant A/B ratio required by the composition of perovskites. prevents formation of CS planes. Defect-ordering in AB03 oxides involves a conservative mechanism in the sense that the vacancies are assimilated into the structure resulting in large supercells of the basic perovskite structure. The type of superstructure formed depends however on the identity of the B-cation. 

Perovskite-related Oxides.—The perovskite-related oxides have been studied extensively in recent years because of the large variety of device applications for which these materials are suited. The interaction between structure, properties, and stoicheiometry is significant at all levels, but here we will discuss only the narrow areas where intergrowth is a dominant structural feature. We will not, therefore, consider solid solutions typified by the Pb(Zr Tii )03 ferroelectrics, and neither will we discuss the structurally complex but stoicheiometric phases related to hexagonal BaTiOj, which includes BaNiOj, which has a simple two-layer repeat in the c-direc-tion, the nine layer BaRuOj, the twelve layer Ba4Re2CoOj2, and the twenty-four layer Sr5Re20ig phase. The crystal chemistry of these phases is treated in detail by Muller and Roy. The materials we shall discuss are the two series of phases A B 0 +2 and A + B 02n+, and the bismuth titanates. Some of the anion deficient perovskites, ABO -x, will be considered in Section 5. 

Some Anion-deficient Perovskite-related Structures.—In this Section we will consider some anion-deficient perovskite phases which can be written as There has... 

Anion vacancy in perovskites is more common than cation vacancy. Unlike the well-known case of W03, anion-deficient nonstoichiometry is not accommodated by the crystallographic shear mechanism, but by assimilation of vacancies into the structure, resulting in supercells of the basic network. The review by Rao et al. (24) contains numerous examples of this kind of behavior. Anion excess has been described in a more limited number of systems. Structural details of this type of compounds can be found in Rao et al. (24) and Smyth (25). 

Rez] Reznitsku, L.A., Thermochemistry of Anion-Deficient Perovskites and Compovmds with Brownmillerite Structiue , Russ. J. Phys. Chem., 64(8), 1197-1199 (1990), translated from Zh. Fiz. Khim., 64, 2228-2231, (1990) (Thermodyn., Calculation, 14)... 

---