Cuprate Superconductors and Related Phases

The copper valence in most compounds lies between the formal values of Cu and Cu. The appearance of superconductivity and the superconducting transition temperature, T, at which the superconducting state appears on cooling, are closely connected with the composition of these solids, which all exhibit considerable degrees oxygen composition variation (Section 8.4). 

There are a number of other structures similar to that of YBajCUjO, especially those in which the Y cations are replaced with lanthanoid ions Ln. In addition, phases with B-site substitution, for instance, YSr Cu CoO, are known, although many of these have structures that are not so obviously perovskite related. The compound YSrCUjCoO., for example, is built from tetrahedra and square pyramids and does not show a simple structural relationship with perovskite. 

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