PHASE-TRANSITION IN BI-BASED SUPERCONDUCTIVE OXIDES EXAMINED BY HRTEM

A Bi-Sr-Ca-Cu-O oxide rapidly cooled from 900°C, at which temperature the specimen is partially molten, consists of a major component of the (2201)-phase and a minor component of the impurity phases. (Ca1-zSrz)3Cu5Oy (z=0.46) and (Ca1-x, Srz)2CuO3 (z=0.09). The grains of both phases have a columnar shape. High-resolution transmission electron microscopy (HRTEM) shows that thin plates of the (2201)-phase are irregularly mixed with the grains of the impurity phase. For the specimen annealed at 500°C in air the electrical resistivity abruptly decreases at the superconducting transition temperature Tc of about 75 K, and this is related to the appearance of a small amount of the (2212)-phase. A characteristic structure found in local areas suggests that a transition state exists between the (2201)-phase and the (2212)-phase. When the specimen is annealed at 850°C the (2212)-phase becomes the major component, and Tc is increased to 87 K. When further annealed at 880°C, the (2223)-phase becomes the major component, and this seems to be initiated by the disproportionation reaction of the (2212)-phase. The resistivity starts to drop at about 112 K. © 1990.