The Perovskit Type

FIGURE 4.38 The geometry, coordinates, unit cell and the coordination polyhedra representations, respectively, for the calcium titanite as perovskite type after Heyes (1999). 

Quantum Nanochemistty— Volume IV Quantum Solids and Orderability 

Over 50 metallic ions can be combined in perovskite structure, the only condition being that the positive and negative charges to be equal in number, such as or M M Xj, so being characterized by the big radius 

The most popular perovskites are the oxides (in majority) followed by fluorides, sulphides, halides and some selenides. 

A special property of perovskites is the supercondictibility, the appearance of the electronic conduction (electronic current) at superior temperatures (also called critical temperatures T) respecting the temperature of (absolute zero) OK. 

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The structure of the perovskite-type lithium ion conductor Li0 29La0 57Ti03 is represented in Fig. 6. The small gray circles depict the lithium ions, the big gray circles the lanthanum ions. These are randomly distributed over the A sites 14 per-... 

Figure 6. Structure of the perovskite-type lithium-ion conductor Li 2yLa057TiO3. The lithium ions (small, gray) and the lanthanum ions (large, gray) are randomly distributed over the A sites, of which 14 percent are vacancies, enabling the lithium ions to be mobile. Titanium forms TiOh octahedra, as shown in yellow. The unit cell is indicated.

Fig. 2 shows the temperature as a function of irradiation time of Cu based material under microwave irradiation. CuO reached 792 K, whereas La2Cu04, CuTa20e and Cu-MOR gave only 325, 299 and 312 K, respectively. The performances of the perovskite type oxides were not very significant compared to the expectation from the paper reported by Will et al. [5]. This is probably because we used a single mode microwave oven whereas Will et al. employed multi-mode one. The multi-mode microwave oven is sometimes not very sensitive to sample s physical properties, such as electronic conductivity, crystal sizes. From the results by electric fixmace heating in Fig. 1, at least 400 K is necessary for NH3 removal. So, CuO was employed in the further experiments although other materials still reserve the possibility as active catalysts when we employ a multi-mode microwave oven. 

In perovskite-type catalysts the formation of the final phase is completed already at 973 K. XRD and skeletal FTIR/FTFIR data for LalCol, LalMnl and LalFel calcined at 973 K evidence that only LalFel-973 is actually monophasic and consists of a perovskite-type phase with orthorombic structure. A perovskite type phase with hexagonal-rombohedral structure represents the main phase of LalCol-973, but traces of C03O4 and La2C05 are also present. In the case of LalMnl-973 two phases have been detected both with perovskite-type structure, one orthorombic and the other rombohedral. The calculated cell parameters of the dominant perovskite-type phase are reported in Table 1 for the three samples. The results compare well with those reported in the literature [JCPDS 37-1493, 32-484, 25-1060] which refer to similar samples prepared via solid state reartion. All the perovskite-type samples are markedly sintered... 

An example for a compound of the perovskite type is LaNiOj. In other com-ponnds of the perovskite type, nickel may be replaced by cobalt or iron, and lan-thannm in part by alkaline-earth metals, an example being Lag 8Sro2Co03. The activity of perovskites toward cathodic oxygen reduction is low at room temperature but rises drastically with increasing temperature (particularly so above 150°C). In certain cases the activity rises so much that the equilibrium potential of the oxygen electrode is established. 

Among the high-temperature superconductors one finds various cuprates (i.e., ternary oxides of copper and barium) having a layered structure of the perovskite type, as well as more complicated oxides on the basis of copper oxide which also include oxides of yttrium, calcium, strontium, bismuth, thallium, and/or other metals. Today, all these oxide systems are studied closely by a variety of specialists, including physicists, chemists, physical chemists, and theoreticians attempting to elucidate the essence of this phenomenon. Studies of electrochemical aspects contribute markedly to progress in HTSCs. 

Superstructures of the perovskite type. Only in one octant have all atoms been plotted the atoms on the edges and in the centers of all octants are the same... 

Uenishi et al. [95] investigated the redox behaviour of palladium at start up in the perovskite-type structure LaFePdOx. An interesting behaviour is reported due to their self regenerative function, which provides high catalytic performances under cycling... 

Uenishi, M., Tanigushi, M., Tanaka, H. et al. (2005) Redox behavior of palladium at start-up in the perovskite-type LaFePdO automotive catalysts showing a self-regenerative function, Appl. Catal. B 57, 267. 

The activities of the perovskite-type oxides are strongly dependent on pretreatment in reducing or oxidizing atmospheres at a.600°C. This was found for other perovskite catalysts as well (1). Reducing pretreatments lead to more active catalysts (Figures 5 and 6). The reason for this is not known, but better binding of CO to the reduced surface is a possible explanation. 

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 7.16 The perovskite-type structure. Small black circles represent the B atom, large grey circles represent O atoms and open circles represent the A atom.

Megaw, H. D. Crystal stmcture of double oxides of the perovskite type. The Proc. Phys. Soc., 1946, Voulme 58, 133-152. 

At this point, it may be informative to present a chronological listing of the different discoveries in oxide superconductors reported prior to 1975. In this listing, Table 5, we present the year that the oxide compound was first reported, then the year in which superconductivity was first observed in the system and the group credited for the discovery. Of particular interest is the compound Ba(Pb1 xBix)Os discovered by Sleight at du Pont in 1975. This oxide material adopts the perovskite-type structure and contains no transition metals. 

Studies of Superconducting Oxides with the Perovskite-type... 

Sugiura, H. and Yamadaya, T., High Pressure Studies on the Perovskite-Type Compound BaBiOs. Physica 139 140B 349 (1986). 

Mizusaki, J., Yamauchi, S., Fueki, K. and Ishikawa, A., Nonstoichometry of the Perovskite-type Lai xSrxCi03 j, Solid State Ionics 12, 1984, 119. 

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