Layered perovskites

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. 

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... 

K. Chondroudis and D.B. Mitzi, Electroluminescence from an organic-inorganic pereovskite incorporating a quaterthiophene dye within lead halide perovskite layer, Chem. Mater., 11 3028-3030,1999. 

The property of copper to take coordinations smaller than six, and especially pyramidal and square planar coordinations which favour again the bidimen-sional character within the perovskite layers. 

Figure 24m HREM image of a characteristic defect, an extra row of atoms appears in the perovskite layer such a defect is interpreted by the existence of a double row of edge sharing Cu04...

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.

Although the full implications of these data are yet to be analyzed, it seems likely that the lattice or size mismatch between the Bi-O and the perovskite layers plays a key role in constituting the... 

The discovery of thallium containing superconductors (4) was another important development. Several superconducting phases exist and consist of intergrowths of rock salt (TI-O) and perovskite layers. They have been reported with zero resistance and Meissner effect up to 125K, i.e., with the highest critical temperatures discovered so far. 

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.

Fig. 13.1. The lattice parameter of a perovskite layer as a function of its cation coordination number. Cations in the BO2 layers are shown on the left, those in the AO layers are shown on the right.

The SNMS depth profile (ion intensity as a function of sputter time) for the matrix elements of a Ba07Sr03TiO3 layer on a silicon substrate with Pt/Ti02/Si02 buffer layers is illustrated in Figure 9.8. Inhomogeneity of the perovskite layer was detected especially for Sr. Furthermore, an interdiffusion of matrix elements of the Ba07Sr03TiO3 layer and of the Pt barrier layer was observed. 

Figure 9.8 SNMS depth profile of the matrix components of a barium strontium titanite (BST) perovskite layer on Si with a Pt/Ti02/Si02 diffusion barrier layer measured using SIMSLAB 410 (FISONS Scientific, iOkeV, Ar+ primary ions). (]. S. Becker and H. ]. Dietze, Int. /. Mass Spectrom. Ion Proc. 197, 1(2000). Reproduced by permission of Elsevier.)...

Depth profiles of matrix elements on Mn- and Co-perovskite layers of fuel cathodes have been measured by LA-ICP-MS in comparison to other well established surface analytical techniques (e.g., SEM-EDX).118 On perovskite layers at a spatial resolution of 100p.m a depth resolution of 100-200 nm was obtained by LA-ICP-MS. The advantages of LA-ICP-MS in comparison to other surface analytical techniques (such as XPS, AES, SIMS, SNMS, GD-OES, GDMS and SEM-EDX) are the speed, flexibility and relatively low detection limits with an easy calibration procedure. In addition, thick oxide layers can be analyzed directly and no charging effects are observed in the analysis of non-conducting thick layers. 

Table 9.15 Comparison of determinations of stoichiometry in thin Ba0,7Sr0 3Ti-, 03 perovskite layer measured by ICP-MS using different instruments and XRF.

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