ABO3 perovskites

Figure 10. Unit cell of ABO3 perovskite structure. (Reprinted with permission from Nature Materials (http //www.nature.com/nmat), ref 128. Copyright 2003 Nature Publishing Group.)...

Figure 1. Schematic structure of ideal ABO3 perovskite.

For the ideal cubic lattice of ABO3 perovskite, A is the larger cation surrounded by eight BC>6 octahedra. In order to estimate the general possibility of perovskite lattice formation from ionic radii, the Goldschmidt tolerance factor (ts) can be used [i] ... 

FIGURE 22.1 The ABO3 perovskite structure (a) AO12 cuboctahedra, (b) BO octahedra. 

It has been reported that even if TCF is related to the tolerance factor (t) in the complex perovskite, differences among compositions with the same value of t can be observed. These differences can be explained by the bond valences of the A- and B-sites in the ABO3 perovskite structure. The bond valence and TCF of PCFNT as well as PCCN and PCMT were investigated to evaluate these relations because the ionic radii of Nb " and Ta " are the same value of 0.64 A at C.N. = 6. Table 22.5 shows the B-site bond valence of PCFNT obtained from... 

Fig. so. ABO3 perovskite structure. The black circles represent the transition metal ions (B ions), the A ions are represented by the shaded circles, and the open circles are oxygen ions. The B—O distance is usually about 2 A. Each B ion is surrounded by an octahedron of oxygen ions. (From Ref. 4SS.)... 

As mentioned in sect. 2.5.1.1 the activity of ABO3 perovskite-type oxides for CH4 combustion is almost not affected by changes in the B-site cations (table 19). The partial substitution of Sr or Ag in the A site (table 21) improved the catalytic activities but not so much as observed for hydrocarbon combustion at low reaction temperatures. (See sect. 2.5.1.2). 

The amount of information available in the literature concerning the use of perovskite catalysts for the water gas shift reaction is very minimal. However it has been shown that some ABO3 perovskites have exhibited significant activity for the reaction. GdFeOs is such an example whose activity is attributed to their p-type semi-conductivity due to a limited number of Fe cations, existing in their crystal lattice and associated with the presence of some cation vacancies [10]. 

Fig. 5. The ABO3 perovskite structure. The lanthanum ion (A) takes position within the octahedra representing the oxygen anion sublattice. These octahedra in turn centre on the manganese (B) sites...

If the ABO3 perovskite structure is cut parallel to the (110) planes, slabs of the composition obtained if these slabs are stacked, an extra... 

The superconductors in class II have the crystal structure A2BO4, which is conventionally called a K2NiF4 compound. A typical class II superconductor is (Lai- cBa c)2Cu04 with Tc - 30 K, discovered by Bednorz and Muller. As seen in Fig. 2, A2BO4 is composed of an ABO3 perovskite unit and an AO unit... 

Anion-deficient nonstoichiometry in ABO3 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 ABO3 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. 

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