Structure evolution in high-T_c Bi-based superconductors with Pb doping revealed by electron microscopy

The structural evolution of a series of Pb-doped Bi-based superconductors, composed mainly of the high-T_c phase, has been studied by transmission, as well as scanning, electron microscopy. It has been observed in all specimens that a minor (2212) phase is often intergrown within a (2223) phase matrix and that impurity phases also coexist. In the specimen annealed for 56 hr at 870°C the (2223) phase contains a Bi-type structural modulation, with q = b*/4.93. In the specimen annealed for 90 hr, besides the Bi-type modulation with q₁ = b*/4.95, a Pb-type modulation with q₂ = b*/7.71 has been observed frequently. In some regions adjoining grains are in a twin relationship around [001], but often the rotation angle is not exactly 90°; i.e., the [100] of one grain is deviated slightly from the [010] of another grain. In the specimen annealed for 95 hr the Bi-type modulation has q₁ = b*/5.00, whereas the satellites caused by the Pb-type modulation become too diffuse to determine q₂. In some local areas the twin planes are not parallel to (001). In some grain boundaries along (001), thin amorphous regions, rich in Ca, are often formed. The relationship between microstructure and superconductivity is discussed.