Single perovskite layers

In contrast to the lead cuprates, most of the thallium cuprates do not require the presence of a rare-earth element for their stabilisation. The 40 K superconductor Tl j Prj.Sr2 Prj,Cu05 6 (Bourgault et al. 1989) seems to be the only one whose structure is stabilised by the presence of praseodymium. The isostructural phase TlSr2Cu05 5 could indeed not be isolated, but was only observed as a mixture with an unknown phase. In this 1201 structure (fig. 22), single perovskite layers [(Sr,Pr)Cu03]oo are intergrown with double rock-salt layers [(Tl,Pb,Sr)202]oo- On the other hand it is... 

Most of the dislocations found have been of this type. In principle, dislocations introducing only a single perovskite slab are possible. In these, the antiphase displacement across the Bi202 layers, i.e. 2ass must still be accommodated, and the smallest Burgers vector for such dislocations would appear to be [a0c]ss. 

Figure 3 The stmcture of different members of the family of the titanates Sr0(SrTi03)m (a) m = 1, (b) m = 2, (c) m = 3. They correspond to the intergrowth of single SrO rock-salt-type layers with multiple perovskite layers (SrTiOsV.

Figure 4 a,b,c Crystal stmcture of cuprates characterized by single rock salt layers (n=l) (a) m=l member, (b) m=2 member, (c) m=3 member (the stmcture of this member which corresponds to the oxide BaPbYSrCusOg differs from the general description by the location of oxygen vacancies in one sheet of the triple perovskite layer). 

Fig. 1. Structure of the oxides La2, A Cu04 (A = Ca, Sr, Ba), an intergrowth of single rock-salt-type layers and single perovskite l ers.

Fig. 3. Structure of La2CaCu20j, intergrowth of single rock salt layer with double oxygen-deficient perovskite layers.

The general formulation of these oxides, (ACuOg.x)m(AO)n, reflects for each of them the number m of copper layers which form each perovskite slab, and the number n of AO layers which form each rock salt-type slab (the AO layers which lie at the boundary of the perovskite slabs and rock salt type slabs can only be counted as for 1/2). Thus all these oxides (2-34) can be represented by the symbol [m,n] in which m,n will be integral numbers. In most of these oxides one observes for one compound only one m and n value, corresponding to single intergrowths. 

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... 

Another closely related family of superconductors is represented by the formula TlBa2Can 1Cun02n+3 (n = 1, 2, 3, 4, 5). They contain single layers of T1 and O atoms that separate the perovskite-like Ba-Cu-Ca-O slabs (24)(25)(26)(27) (Figure 6). Distortions in the Tl-O sheets are also found in these compounds (26)(27). Note that if these phases were stoichiometric, copper would always have a formal oxidation state of greater than two. Therefore, the chemical composition of this homologous series allows the existence of holes in the copper-oxygen sheet. 

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