Layered perovskite structures

In the materials that follow, the structures are all layered. This structural feature has lead to a description of the doping in terms of charge reservoirs, a different approach to that described previously, and which is detailed below. Structurally the phases are all related to the perovskite-layered structures (Figs. 4.27, 4.28, 4.29, and 4.30). The similarity can be appreciated by comparison of the idealized structures and formulas of some of these materials, Bi2Sr2CuOg... 

As with the other stmctures described, the symmetry of mixed cation materials is often lower than that implied by the idealised stmctures, due to distortions of the metal-oxygen octahedra in the perovskite slabs and alterations in the cation arrangements in the inter-perovskite layers. Structures are often given similar but different space groups, and in some cases it is deemed best to described the stmctures as incommensurate modulated forms. 

This method of approach, when applied to the above mentioned compounds leads to a Na2Ta306F5 structure in which n = 3 (n denotes the number of layers) if the perovskite positions remain vacant, as shown in Fig. 36 (a). The Na4Ta5OioF9 phase corresponds to n = 2.5, which leads to the proposed structure consisting of two types of alternating layers, characterized by n = 2 and n = 3, as shown in Fig. 36 (b). The central positions of the perovskite layers remain vacant in this structure as well. 

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. 

Alternate layers can be occupied by two different kinds of metal atom, then every pair of the face-sharing octahedra contains two different metal atoms this is the ilmenite type (FeTi03). Ilmenite is, along with perovskite, another structure type for the composition AiiMiv03. The space for the A2+ ion is larger in perovskite. Which structure type is preferred can be estimated with the aid of the ionic radius ratio r(A2+)/r(02-) < 0.7 ilmenite... 

Kato, K. Zheng, C. Dey, S. K. Torii, Y. 1997. Chemistry of the alkoxy-derived precursor solutions for layer-structured perovskite thin films. Int. Ferro. 18(l-4) 225-235. 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

Fig. 6. a) The layer-structure of an ordered perovskite Sr2(BCr)Os vertical to the hexagonal c-axis (idealized description) b) Schematic illustration of a- and 7t-... 

The description of this structure is more complicated than that of Ba2YCu3Ox. There are six layers in the unit cell of this structural type and they can be viewed in two quite different ways. In the first interpretation, we divide the six layers into two blocks of three layers each, the first being (AX)0(BX2)C(AX)0 and the second (AX)C(BX2)0 (AX)C. These layers and these sequences are typical of perovskite and, therefore, in this description the structure is considered to be made of two perovskite blocks related to one another by a shift of origin of t = (l/2)(a + b). We may also regard the structure, however, as containing alternate blocks of perovskite (layers (BX2)0 c) and rock salt (layers (AX)co(AX)oc). As before, the unit cell is made of two... 

Figure 8 Idealized structure of Bi2Sr2CaCu208 (2212) showing perovskite layers separated by Bi202 lamellae.

The fundamental structure of Bi2Sr2CaCu20g (or the 2212 phase), ignoring the modulations, is orthorhombic, with a = 5.39 A, = 5.41 A and c = 30.8 A. A schematic diagram of the structure is shown in figure 6.4(a). It consists of a Bi202 double layer and a perovskite layer containing two Cu02,... 

Figure 7.19 Schematic representation of the structures of (a) La2Cu04, (b) Bi2Sr2CuOg and Tl2Ba2CuOg, (c) TlCaBa2Cu207 and (d) Bi2CaSr2Cu20g and Tl2CaBa2Cu20g, showing intergrowth of rock-salt and perovskite layers. Oxygens are shown as open circles and Bi and T1 by crosses.

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