Electron Microscopy Study of Pb-Substitution in Bulk Bi-Sr- Ca-Cu-O Superconductors.

We present the results of an electron microscopy study of Pb- substitution in bulk Bi-Sr-Ca-Cu-O superconductors. We observe grains with c-axis lattice parameters of 30.8angstroms (2212 phase), 36.7angstroms (2223 phase) and 43.2angstroms (2234 phase) containing no intergrowth defects. The existence of two separate incommensurate superstructures, Q sub 1 and Q sub 2, which were previously observed in Pb-doped bulk and single crystals, have been confirmed in this study. It is found that superlattice reflections from the Q sub 1 -modulated structure in the 2223 phase are much weaker than those from the Q sub 2 -modulation, whereas the diffraction intensities of the Q sub 1 -and Q sub 2 -modulations in the 2212 phase are similar. We also show that the periodicity along the b-axis of the Q sub 2 -modulation increases with increasing Pb-substitution, contrary to the implications in the previously suggested structural model.