Perovskite-Related Structures and Series

6 CUPRATE HIGH-TEMPERATURE SUPERCONDUCTORS 8.6.1 Perovskite-Related Structures and Series 

The important and widely studied copper-oxide-derived high-temperature superconductors, known as cuprate superconductors, are basically insulators. Doping converts these into metallic materials, many of which are superconductors over rather more restricted composition ranges. Several of these materials have already been discussed La2Cu04 and Sr2Cu02F2 (Section 4.3.3), La2 A.SrxCu04 (Section 8.5.1), and Nd2, Ce,Cu04 (Section 8.5.2). 

In the materials that follow, the structures are all layered. This structural feature has lead to a description of the doping in terms of charge reservoirs, a different approach to that described previously, and which is detailed below. Structurally the phases are all related to the perovskite-layered structures (Figs. 4.27, 4.28, 4.29, and 4.30). The similarity can be appreciated by comparison of the idealized structures and formulas of some of these materials, Bi2Sr2CuOg 

In these and the other cuprate superconductors, the part of the structure that leads to superconductivity is the slab of Cu02 sheets. When more than one sheet is present, they are separated by cation layers, Q (usually Ca or Y) to give a sequence Cu02-(Q-Cu02) i, which forms the superconducting layer in the material. The index n is the total number of Cu02 layers in the phase, which is equal to the formula number of Cu atoms present (Fig. 8.5). 

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