Perovskite layer structure

Perovskite Materials. Our studies of these materials are still in their infancy. An important feature of the perovskites is the ability to form many phases through shear planes where layers of a metal oxide can be inserted between multiple perovskite structure layers. Our work has shown that probe ions can be used to watch this process and we have been able to show that site selective laser spectroscopy is sensitive to all of the phases with a high sensitivity to even small concentrations. 

Perovskite structures, 5 598 Perovskite-type layered superconductors, 23 852... 

The structures of ternary oxides such as spinels, perovskites, pyrochlores, layered cuprates (high-7 c superconductors), and other lamellar oxides are fascinating subjects by themselves and are beyond the scope of the present discussion. 

A. Kudo, H. Kato, S. Nakagawa, Water splitting into H2 and O2 on new Sr2M207 (M — Nb and Ta) photocatalysts with layered perovskite structures Factors affecting the photocatalytic activity, J. Phys. Chem. B 104 (2000) 571-575. 

Kudo, A., Kato, H., Nakagawa, S. 2000. Water splitting into Hj and Oj on new StjMjO, (M=Nb and Ta) photocatalysts with layered perovskite structures factors affecting the photocatalytic activity. J Phys Chem B 104 571-575. 

Takata, T., Shinohara, K., Tanaka, A., Kara, M., Kondo, J.N., Domen, K. 1997a. A highly active photocatalyst for overall water splitting with a hydrated layered perovskite structure. J Pho-tochem PhotobiolA Chem 106 45 9. 

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. 

Figure 3 Comparison of the structures of perovskite (ABXS), Ba2YCu307, and Ba2YCu306. The scheme at the bottom of figure shows the description of the three structures layer by layer.

In this large class of materials the blocks R = m (AX) with the rock-salt structure are made by two or more layers of type (AX) which may be identical to each other or have different chemical compositon. The blocks P = (BX2)oc (n-1) [(AX)c o(BX2)o c] with the perovskite structure may have different values of n, and the layers (AX), sandwiched between layers (BX2), may or may not be defective. The important homologous series with the rock salt-perovskite structure are listed in the scheme of Figure 9where they are compared with each other and with the basic structure of perovskite. 

All the structures belonging to this system have the sequence represented in the scheme of Figure 9. The rock-salt slab is made of four layers of type (AX), followed by slabs of variable thickness having the perovskite structure. Since the number of layers (AX) is even, every one of these structures is made of two identical halves shifted by t = l/2a + l/2b. The Tl-Ba system is isomorphous with the Bi-Sr system. However, compounds Bi2Sr2Can 1Cun02n+4 have superstructures whose atomic configurations have not been completely clarified. 

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 1.9 (a) The perovskite structure. Without the large A atom at the body centre position, the structure becomes that of cubic ReOj (b) The K2Nip4 structure consisting of rocksalt (KF) and perovskite (KNiFj) layers. The NiFg octahedra share equatorial corners restricting the Ni-F-Ni interaction to the xy-plane. 

An example of how such restrictions work in three dimensions is provided by the perovskite structure shown in Figs 10.3 and 10.4. Crystals of BaTiOa (23759) are composed of an alternation of the BaO and Ti02 layers shown... 

The parent perovskite structure shown in Fig. 10.4 consists of alternating layers of composition AO and BO2, as for example in BaTiOs (23759) and CaTiOs (62149). It is also possible to have several AO layers between each BO2 layer providing each AO layer is sheared by half a unit cell from the adjacent AO layers, as shown for La2Ni04 in Fig. 12.1. This permits a wide range of structures with an even wider range of compositions to be prepared. Which compositions are possible depend on how well the structure can accommodate the bonding requirements of the atoms A and B. 

The first breakthrough superconductors were formulated as La2-rBarCu04 ft (jt < 0.2. unspecified but small) and have the tetragonal, layered K2NiP4 perovskite structure. They had a critical temperature of about 35 K.42... 

Fig. 16.2 Idealized structures of some perovskite-related layered oxides and their fluoride relatives.2)...

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