Chemical and structural relationships in high-Tcmaterials

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Abstract
The following four features of the high-Tc copper oxides are discussed. (i) They have intergrowth structures consisting of superconductive layers of fixed oxygen concentration and 'inactive' layers of variable oxygen concentration. The intergrowth structure imparts anisotropy, internal electric fields and bond-length mismatch, all of which influence the chemistry, structure and superconductive properties of these materials. (ii) Superconductivity occurs in a narrow compositional range in which the correlation splitting of a sigma *(x2-y2) band and its associated short-range spin fluctuations are retained through a transition from antiferromagnetic semiconductor to normal metal. (iii) The interaction Cu3++O2- to or from Cu2++O- is not biased strongly to the right or to the left. (iv) The width of the conduction band is at the narrow-band limit for a mixed-valent conductor; perturbations that transform mobile charge carriers from itinerant electrons/holes to small polarons suppress the superconductivity. A final assessment is made of the classes of theoretical suggestions that have been put forward to explain the high-Tc phenomenon.