SOME CHEMICAL AND STRUCTURAL EFFECTS ON THE PROPERTIES OF HIGH-T-C SUPERCONDUCTORS

The history of superconductivity, including superconductivity in the h igh-T-c cuprates, is reviewed very briefly, and the differences betwee n conventional superconductors and the high-T-c cuprates are summarize d. The basic crystal structures of the major series of high-T-c cuprat es are described and compared. The relation of structures to supercond ucting properties is reviewed with an emphasis on the orthorhombic-tet ragonal transition in (La-2-x Sr-x)CuO4; the corresponding transition, and also the transition to the low-temperature tetragonal phase in (L a2-xBax)CuO4; and the effects of oxygen vacancies, oxygen-vacancy orde ring, frozen-in disorder, and occupation of the off-chain O(5) sites i n YBa2Cu3O7-delta. The effects of chemical substitutions of lanthanide elements on the La/Sr sites in (La2-xSrx)CuO4 and on the Y sites in Y Ba2Cu3O7-delta, and of 3d elements and sp elements on the Cu sites in both systems are reviewed. The difference between the effects of the l anthanide substitutions, particularly Pr, in the two systems are consi dered. Major properties of the Bi-, Tl-, and Hg-cuprates, are describe d briefly. A comparison of the high-T-c cuprates with noncuprate oxide superconductors is given.