Perovskite structure cation deficient

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. 

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

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. 

Defects in perovskite oxides can be due to cation vacancies (A or B site), amon vacancies or anion excess. Cation-deficient oxides such as A,WOj give rise to oxide bronze structures, W03 itself representing the limiting case of the A-sile deficient oxide A-site vacancies are seldom ordered in these metallic systems. B-site vacancies are favoured in hexagonal perovskites and ordering of these vacancies gives rise to superstructures in some of the oxides. 

Partial substitution of A and B ions is allowed, yielding a plethora of compounds while preserving the perovskite structure. This brings about deficiencies of cations at the A-or B-sites or of oxygen anions (e.g. defective perovskites). Introduction of abnormal valency causes a change in electric properties, while the presence of oxide ion vacancies increases the mobility of oxide ions and, therefore, the ionic conductivity. Thus, perovskites have found wide apphcation as electronic and catalytic materials. 

The family of bismuth oxides of the general formula Bi B -,Oj (B=Ti, Nb and W), first described by Aurivillius, may be regarded as B-cation deficient perovskites. Typical members of this family are Bi2WOfi, BiaTlNbOg and Bi4Ti30i2- These phases adopt layer structures consisting of PbO-like (BiaOj) layers which alternate in the orthorhombic... 

Although the stacking sequence of the AO3 layers in these compounds is the same as in the previous materials, in these cation-deficient phases the central octahedron of the triplet groups does not contain any B cations. Because of this the A B jOj structures can be conveniently considered to consist of layers of comer-sharing cubic perovskite sttucture-type (n-1) octahedra in thickness (Figure 3.17a-c). It is seen that on moving from one cubic perovskite layer to the next, the rows in the sheets of apex-linked octahedra are displaced or shifted laterally one relative to the other as in the stoichiometric A B Oj phases in the previous section. 

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