SURFACE AND BULK DEFECTS IN HIGH-T(C) SUPERCONDUCTING OXIDES OBSERVED BY HIGH-RESOLUTION ELECTRON-MICROSCOPY

Extended defect structures are well known in superconducting oxides, the most common being intergrowths of different members of a structrual series such as T12Ba2Can-1CunO2n+4 or YBa2Cun+2O2n+6. Such intergrowths may be either ordered or disordered, but invariably consist of related structural units which come into contact at the layer which they have in common. More complex arrangements can however occur, when either totally different phases intergrow, or alternatively, when similar structural units are forced to come into contact at a point where their structures differ. The most common example of the former is found at the surfaces of many superconducting materials: the surface phases which may develop usually comprise simple oxide materials which are not superconducting and may have a large effect on the intergranular critical currents in real applications. Examples of such phenomena on La2CuO4, YBa2Cu3O7-delta and Tl2Ba2CaCu3O10 will be discussed. The second type of intergrowth, which exists in compositions from (Pb,Cu)Sr2(Ca,Y)Cu2O7 to YBa2Cu3O7, gives rise to a psuedo-solid solution between the two end members, where the interlayer oxygens separating the perovskito-like blocks undergo a co-operative distortion to accomodate the different coordination requirements of the two cations. The structural aspects of these features will be discussed, and the reliability of the structural data obtained from electron diffraction and high resolution electron microscopy will be assessed.