Perovskite structure blocks

In these compounds, the blocks R of expressions (9) and (10) have m - 1, i.e., the rock-salt blocks are monolayers (AX). Important materials with the perovskite structure are represented in the scheme of Table 1, where they are compared to each other and to the general structural type discussed in the previous section. 

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. 

In contrast to the situation in the 3-dimensional perovskite structure, two distinct sites are available for Ln and Ba cations, in this quasi-2-dimentional crystal structure. A 12-coordinate site (Ba/Lal) is situated within the perovskite-like blocks, and a 9-coordinate (Ba/Ln2) is situated on the edge of the blocks referred to as the rock-like layer. The availability of these two sites introduces an additional parameter, which has an influence over the properties of this phase, namely, the degree of cation ordering on the Ba/Ln sublattice [6]. Therefore, it is worthy that the pure phase containing Mn ion only is... 

Figure 93 Basic structural types of cuprate superconductors, (a) Perovskite structure (cubic) (b) Infinite- layered structure (tetragonal) (c) Rocksalt prototype structure (reduced cell) (d) Composite layer between infinite layer and rocksalt building blocks.

The perovskite stracture tolerates relatively large nonstoichiometries and substitutions, while a number of perovskite-related stmctures (with perovskite building blocks) are formed when excesses, deficiencies or substitutions beyond the solubility limits lead to ordered defects and new phases. These comprise, for instance, oxygen deficiency (brownmillerite, ABO2.5), and A excess (K2NiF4-type structure, A2BO4) and ordered intermediate phases. 

In the perovskite structure with 3d-block transition-metal atoms M, the dominant interactions between d-like orbitals centered at neighboring M atoms are the (180° - 0) M—0—M interactions. The spin-independent resonance integrals describing charge transfer between M atoms at Ri and Rj are... 

Thus, today Ruddlesden-Popper phases include complex oxides eontaining metals from groups 1,2, 13, 14 and 15 as well as transition (d- and 4f-) elements. The general formula of such oxides can be written as A0(AB03)n, where A means alkaline, alkali earth or rare earth element, while B is d-element, Al, Ga, In, Pb or Bi. This formula shows the main structural feature of this class of the layered perovskite-like eompoimds, that is the intergrowth of the perovskite blocks AMO3 (P) and rock salt blocks AO (RS) in a consequence -Pn-RS-Pn-RS-. In case of n = 00 the perovskite structure itself is obtained. Pigure 1 shows schematically the structure of the Ruddlesden-Popper phases (n=3) as eompared to the other types of the layered perovskite-like compounds. 

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... 

Following the above discussion, it is now possible to define a general structural type built with alternating blocks having the perovskite and the rock salt structures, according to the scheme ... 

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