Cuprate superconductors layered perovskite structures

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.

Other high-temperature superconductors can be described in similar fashion, e.g. Tl2Ca2Ba2Cu30io (containing Tl, Ca and Ba centres) is composed of layer sequence 27.4. The non-Cu02 oxide layers in the cuprate superconductors are isostructural with layers from an NaCl structure, and so the structures are sometimes described in terms of perovskite and rock salt layers. 

The crystal chemistry of the rare-earth cuprates is discussed in chapter 188 by B. Raveau, C. Michel and H. Hervieu. These authors noted that all of the hiTc superconductors are derived from the perovskite structure. This is done by disconnecting the CuOe octahedra of the perovskite structure along one direction so then an infinite number of CUO2 layers are formed. These layers are responsible for the superconductivity. Raveau et al. note that the rare-earth cations are not directly responsible for the appearance of superconductivity, but by virtue of their large size and trivalent character they help stabilize these layered structures. 

The non-Cu02 oxide layers in the cuprate superconductors are isostructural with layers from an NaCl structure, and so the structures are sometimes described in terms of perovskite and rock salt layers. 

Among the high-temperature superconductors one finds various cuprates (i.e., ternary oxides of copper and barium) having a layered structure of the perovskite type, as well as more complicated oxides on the basis of copper oxide which also include oxides of yttrium, calcium, strontium, bismuth, thallium, and/or other metals. Today, all these oxide systems are studied closely by a variety of specialists, including physicists, chemists, physical chemists, and theoreticians attempting to elucidate the essence of this phenomenon. Studies of electrochemical aspects contribute markedly to progress in HTSCs. 

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. 

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... 

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