Structure ABO3 perovskite

The perovskite structure, ABO3 (where A represents a large cation and B a medium-size cation) is adopted by many solids and solid solutions between them can readily be prepared. Vacancy-containing systems with the perovskite structure are of interest as electrolytes in solid-state batteries and fuel cells. Typical representatives of this type of material can be made by introducing a higher valence cation into the A sites or a lower valance cation into the B sites. 

It should be mentioned that oxygen vacancies are often formed in the perovskite-type structure ABO3 in cases where the B atom is a transition metal that readily exists in more than one oxidation state. 

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.

CrFs has the ReOa-structure, which results from the perovskite structure ABO3 by removing the A-cations. 

The crystal structure of the 1-2-3 superconductor, YBazCusOy- is depicted in Figure 10.8. Figure 10.8(a) depicts only the positions of the metal atoms. If we discuss it in terms of the perovskite structure ABO3, where B=Cu, the central section is now an A-type perovskite unit cell and above and below it are also A-type perovskite unit cells with their bottom and top layers missing. This gives copper atoms at the unit cell corners and on the unit cell edges at fractional coordinates A and Ys. The atom at the body-centre of the cell (i.e., in the centre of the middle section) is yttrium. The atoms in the centres of the top and bottom cubes are barium... 

Figure 10 Perovskite structure, ABO3 (large cross hatched circle is A, small shaded circles are B)...

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... 

The structure of perovskites of the general formula ABO3 (e.g., CaTi03) is related to the Re03 structure, where the B atoms (Ti in CaTi03) take the place of Re and the A atoms (Ca in CaTi03) are at the center of the cubic... 

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.)... 

Certain cations comparable in size with form c.p. layers AO3 which can be stacked in various c.p. sequences. Smaller cations can then occupy the octahedral holes between groups of six 0 ions to form structures of the type A B 03 . Some of these structures have been described in Chapter 4, and it was noted in Chapter 5 that these structures may alternatively be described in terms of the way in which the BOg octahedra are linked together. Only vertices and/or faces are shared, and the extent of face-sharing is indicated in Table 13.5. We shall deal in detail only with the simplest of the c.p. ABO3 structures, the perovskite structure. The structure of hexagonal BaTi03 is compared with perovskite in Fig. 13.1, and in Fig. 13.2 we show sections through the 5-, 9-, and 12- layer structures to illustrate the relations between the BOg octahedra. 

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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