Polytypoidic structures in high temperature oxide superconductors

High resolution transmission electron microscopy has been used to reveal the cationic structure of the high temperature oxide superconductors. Of the many new systems discovered recently, the BiSrCaCuO, TlBaCaCuO have been extensively studied. Compositional variations are accommodated by the formation of polytypoidic stacking units whose cationic stoichiometries are in specific fixed ratios. Examples from the Bi cuprate bulk superconductors and Y-Ba-Cu-O thin films are presented. Identification of the different polytypoids, each of which has a specific transition temperature, Tc, can shed light on the electrical properties of the mixed bulk or thin films. Polytypism and polytypoidism also provide a unified approach to the structural and chemical analysis of these oxide superconductors. © 1990.