SOME CHEMICAL AND STRUCTURAL EFFECTS ON THE PROPERTIES OF HIGH-Tc SUPERCONDUCTORS¹

The history of superconductivity, including superconductivity in the high-Tc cuprates, is reviewed very briefly, and the differences between conventional superconductors and the high-Tc cuprates are summarized. The basic crystal structures of the major series of high-Tc cuprates are described and compared. The relation of structures to superconducting properties is reviewed with an emphasis on the orthorhombic-tetragonal transition in (La2−x Srx)CuO4; the corresponding transition, and also the transition to the low-temperature tetragonal phase in (La2−xBax)CuO4; and the effects of oxygen vacancies, oxygen-vacancy ordering, frozen-in disorder, and occupation of the off-chain O(5) sites in YBa2Cu3O7−δ. The effects of chemical substitutions of lanthanide elements on the La/Sr sites in (La2−xSrx)CuO4 and on the Y sites in YBa2Cu3O7−δ, and of 3d elements and sp elements on the Cu sites in both systems are reviewed. The difference between the effects of the lanthanide substitutions, particularly Pr, in the two systems are considered. Major properties of the Bi-, Tl-, and Hg-cuprates, are described briefly. A comparison of the high-Tc cuprates with noncuprate oxide superconductors is given.