Perovskite like structure

F can he suppressed hy the high site symmetry of the central atom In many perovskite-like structures of the ABO3 type the lone pair of the B-cat-ion leads not to a structural distortion. In CsPbF3 under ambient conditions no lone-pair activity observed [27], but upon cooling a phase transition is observed that leads to less symmetrical surrounding of Pb by fluoride [28]. 

Keywords superconductors, perovskite-like structures, negative thermal expansion. 

The simplest model for a tungsten bronze, M WOg, is to regard it as a host W03 structure in which M atoms have been introduced interstitially. For the simplest case, which leads to a perovskite-like structure, the unit cell can be represented as in Figure 1, with a tungsten atom at the center, six oxygen atoms at face centers, and eight interstitial sites at the cube corners more or less occupied by alkali atoms. When these sites are completely empty, x in M WOg is zero and the structure resembles that of WOa except that the tungsten atoms... 

Sodium tungsten bronzes, Na W03, with the perovskite-like structure [384 386] have been investigated as oxygen cathode materials in acid solutions, but they have to be promoted by traces of platinum on the surface [386-389],... 

In the Sr3peRu07 and Sr4FeRuOs phases with a perovskite-like structure, Fe + and Ru + ions are randomly distributed over the six coordinate (octahedral) sites. What type of magnetic interactions might be expected in these oxides ... 

FIGURE 14.9 Ideal tetragonal perovskite-like structures of the members of the (a) Rud-dlesden-Popper, (b) Aurivillius, and (c) Dion-Jacobson series. 

The 1-2-3 superconductor has a perovskite-like structure (7.33a,c). There are systematic oxygen atom vacancies in the unit cell compared to a stack of simple perovskite unit cells (Fig. 7.33b). These occur between adjacent copper atoms in the chains along the c axis. The vacancies are in the yttrium atom plane. There are also vacancies between copper atoms along the a axis in the copper-and-oxygen planes... 

X-ray studies have also been made of a series of complex oxides with tetragonal or pseudotetragonal symmetry which all have the a dimension of the unit cell close to 3-8 A, and are based on sequences of Bi202 layers of the same type as in Fig. 20.6, interleaved with portions of perovskite-like structure. They are summarized in Table 20.5. and illustrated in Fig. 20.5. The simplest member of the family is at present represented only by compounds in which part of the 0 is replaced by F. 

Two representatives of the NaCl structure type are diown in Table 8(b), namely InCl and lijTiOa, with very different types of qtlitting and different possibilities for the deformation of the octahedra. A further perovskite-like structure (index 64) is listed below ... 

Tab. 3.4 Metallic radii (A), total energy differences A (kJ/mol) between different atomic orderings relative to a random occupation, and the corresponding lattice parameters a (A) of MFesN in the perovskite-like structure type.

Galy, J. and Carpy, A. (1974) Defects and diffusion paths in perovskite-like structures ABO3+. Philos. Mag., 29 (5), 1207-1211. 

The important role of the catalyst oxygen in the combustion of soot in gas mixtures with NO and O2 was also reported for La2 K NiMn06 catalysts with ABOs-type perovskite-like structure [34]. It was observed that the partial substitution of K at A site leads to an increase of the concentrations of high-valence... 

However, besides lanthanum other rare earth elements showed photocatalytic activity after their incorporation in titanates. Among them, neodymium incorporated in different perovskite-like structures (e.g., NaNdTi04 [86,87], LiNd-TiP4 [87], HNdTi04 [ ]> Bi345Ndo.85Ti30i2 [8 ], and A2Nd2Ti30io —Ei, Na,... 

The superstoichiometric compounds TijAlC, x > 1 ([TiAl3C6Ti9 + C] clusters) and Ti-enriched compounds in the perovskite-like structure were studied. The latter are a complete phase (TijAlC) and a C-defect phase (TisAlCi-J described by the clusters [TiAl4C2Tig], [TiA CoTig]. 

Materials design problems of perovskite-like structures... 

The structure of the ceramic superconductors is basically a sandwich in which CUO2 planes, which carry the supercurrent, are surrounded by perovskite-like structures containing the rest of the atoms. This layered structure along with the compositions of YBa2Cu307 and related HTSC systems is indicated in Table 26.4. 

---