Structural principles of high-Tc superconductor ceramics

Perovskite-related ternary metal oxide phases have been well studies in the past New, additional interest in them arose following the discovery of superconductivity with high Tc in certain alkaline earth - rare earth (RE) cuprates. The structural chemistry of these oxides in characterized by specific functions units: a matrix of 1 to 4 [CuO2]-2+δ sheets separated by Ca or Re elements and intercalation units such as CU-O chains and Tl or Bi oxide layers or double layers which serve as oxygen hosts and thus as electron acceptors, charging the cuprate layers to become conductors and, by a mechanism as yet unclear superconductors. Counter-ion doping [e.g., La1-xSrx in the LSCO and n-doping (Ce in T phase)] also occur. There are interesting phenomena such as twinning and long-period modulation in some phases. In this paper these phenomena and their implications concerning improved Tc and jc are reviewed in the framework of a systematic classification. Support by DOE and SDI-O under subcontract from IGC