PEROVSKITE STACKING IN THE STRUCTURES OF THE HIGH-TEMPERATURE CUPRATE SUPERCONDUCTORS

Most of the known cuprate superconductors belong structurally to one family and are closely related to each other. Ignoring odd structural details and considering the idealized structures only, most of them adopt a Ruddlesden-Popper (R-P) or an oxygen-deficient R-P structure with or without intergrowth layers which are composed of either a rock-salt type atomic arrangement, chains, bare cations, or a mixture of them. The rock-salt layers found in R-P structures are simply the result of the stacking of perovskite unit cells. Sometimes a fluorite type structural unit can be found, but the structures containing fluorite blocks may also be considered to be of the R-P structure type with minor readjustment of anions in the rock-salt layers to stabilize the structure. A general formula can be used to describe the basic structures of all the cuprate superconductors, even those having fluorite units.