Structure of perovskite

The mineral CaTiC 3 is named perovskite, the structure of which is related to a vast number of inorganic crystals, and it plays a central role in the development of the inorganic functional materials. 

Perovskite crystallizes in the cubic space group — Pm3m. The Ti4+ ions are located at the corners of the unit cell, a Ca2+ ion at the body center, and O2- ions at the mid-points of the edges this so-called A-type cell is shown in Fig. 10.4.1(a). When the origin of the cubic unit cell is taken at the Ca2+ ion, the Ti4+ ion occupies the body center and the O2- ions are located at the face centers this B-type unit cell is shown in Fig. 10.4.1(b). 

Each Ca2+ is thus twelve-coordinated and each Ti4+ six-coordinated by oxygen neighbors, while each O2- is linked to four Ca2+ and two Ti4+ ions. As expected, it is the larger metal ion that occupies the site of higher coordination. Geometrically the structure can be regarded as a ccp of (O2- and Ca2+) ions, with the Ti4+ ions orderly occupying of the octahedral interstices. 

The basic perovskite structure ABX3 forms the prototype for a wide range of other structures related to it by combinations of topological distortions, substitution of the A, B and X ions, and intergrowth with other structure types. These compounds exhibit a range of magnetic, electrical, optical, and catalytic properties of potential application in solid state physics, chemistry, and materials science. 

Many ABX3 compounds have true cubic symmetry, whereas some owing to strain or to small departures from perfect cubic symmetry have appreciably distorted atomic arrangements. In the idealized structure, a simple relationship exists between the radii of the component ions, 

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Figure 21.3 Two representations of the structure of perovskite, CaTi03, showing (a) the octahedral coordination of Ti, and (b) the twelve-fold coordination of Ca by oxygen. Note the relation of (b) to the cubic structure of Re03 (p. 1047).

The relationship between the structures of perovskite and Ba2YCu3O70 illustrated in Figure 1, can be easily understood if we represent the sequence of layers in the two cases in the following way. 

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.

On the other hand, before and/or after a layer (AX) there may be a layer of type (BX2) or one of type (AX). This last case is possible because (AX) is characteristic of both the perovskite and the rock salt structures and, therefore, is structurally coherent with both (BX2) and (AX). A sequence. .(AX)c o(AX)o c... can consequently substitute the single layer (AX)co, thus increasing the thickness of the rock salt monolayers (AX) present in the structure of perovskite. In this way, we may derive the structure of A2BX4 (K2NiF4 - type) from that of perovskite by substituting each (AX)C 0 in expression (6) with bilayers... 

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. 

Figure 5.21. The structure of perovskite (CaTiCh). (a) The packing layers are identified (Ti, small O, larger dark Ca, large light), (b) A cube showing the cubic unit cell showing the TiC>6 octahedron and the cube formed by Ca atoms, (c) A cubic arrangement of TiC>6 octahedra with Ca2+ at the center.

Figure 12.3 Crystal structure of perovskite-type hydride, KMgH3.

The structure of perovskites of the general formula ABO3 (e.g., CaTi03) is related to the Re03 structure, where the B atoms (Ti in CaTi03) take the place of Re and the A atoms (Ca in CaTi03) are at the center of the cubic... 

Most HTSC oxides have the structure of perovskite (though some of them have the spinel-type structure), which pertains to more than 35 structural classes [14], and includes more than a hundred typical unit cells [15]. Along with cuprates, certain bismuthates also exhibit HTSC properties. For fundamental studies of superconductivity in oxides, both the absolute Tc values and the variety of properties and structures are essential. Therefore, the titanium compounds with relatively low Tc values are also actively studied. 

Figure 1. Structure of Ba2YCu307.o (right) and its relationship to the structure of perovskite (left). The atoms X are the oxygen atoms of perovskite that do not exist in the structure of Ba2YCu307,o. As a consequence of this elimination, the typical chains are formed on the basel plane of the superconductor, and the atoms Cu(2) assume fivefold, pyramidal coordination.

Fig. 7.33 ta) Unit cell of the 1-2-3 superconductor, orthorhombic, space group Pmmm. Qne-dimensional CuOi chain.s run along the h axis, and two-dimensional CuO layers lie in the ah plane, (bl The cubic structure of perovskite. SrTiOj. Three unit cells are shown slacked vertically, (c) The unit cell of the 1-2-3 superconductor in the context of the surrounding crystal. Copper atoms arc surrounded either by five oxygen atoms in a square pyramid or four oxygen atoms in a square plane. [From Holland, G. F. Stacy, A. M. Acc. Cliem. Res. 1988. 2J, 8-15. Reproduced with permission.)... 

Y. Teraoka, M. Yoshimatsu, N. Yamazoe and T. Seiyama, Oxygen-sorptive properties and defect structure of perovskite-type oxides, Chem. Lett. (1984) 893-896. 

Fig. 8.20. Clinographic projection of the unit cell of the (idealized) cubic structure of perovskite, CaTi03.

The structure of perovskite type is the archetype of numerous synthetic and natural compounds with stoichiometry ABX and their derivative structures. The latter are formally obtained from the archetype through mechanisms such as anion or... 

Figure 9 Structures of perovskite lithium ion conductors (a) (Li, La)Ti03 (b) LiCai es o.35Tii.3Nbi 7O9. ((T800°c = 3.1 x 10 Scm Ea = 0.71 eV). In (a), the green and blue circles denote La and Li, respectively. In (b), the green circles denote Li and Ca in the GdFe03 structure. (Reproduced by permission of Royal Society of Chemistry)...

Liu D-J, Krumpelt M (2005) Activity and structure of perovskites as diesel-reforming catalysts for solid oxide fuel cell. Int J Appl Ceram Technol 2 301-307... 

In Section 3.3.1, we smnmarize available structural information about crystal structures of perovskite-type solid solutions in RGa03-R Ga03 pseudo-binary systems. The literature data are supplemented by original data presented here for the first time. 

To add predictive power to the descriptor approach, researchers have attempted to develop activity descriptors based on material properties that can be intuitively determined prior to material synthesis. One of the popular approaches uses parameters based on electronic structure. For example, Bockiis and Otagawa studied the correlation between catal3riic activity and the electronic structure of perovskites [9]. They concluded that... 

Figure 15.10. Schematic structure of perovskite oxynitride (BaNb02N) [91]. (Reproduced...

Wachowski, L. (1986) Influence of the method of preparation on the porous structure of perovskite oxides. Surf. Coat TechnoL, 29 (4), 303-311. 

Clarke, S.J., Hardstone, K.A., Michie, C.W., and Rosseinsky, M.J. (2002) High-temperature synthesis and structures of perovskite and n = 1 Ruddlesden-Popper tantalum oxynitrides. Chem. Mater., 14, 2664-2669. 

Figure 25.6 Structure of perovskite-type oxides with the generai formula of ABO3 [45].

Fierro, J.L.G. Composition and structure of perovskite surfaces. In Properties and Application of Perovskite Type Oxides, Tejuea, L and Fierro, J.L.G. Eds. Chemical Industries Dekker, New-York. Vol. 50, 1993, pp. 195-214. 

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