Electron microscopy study of Pb-substitution in bulk Bi–Sr–Ca–Cu–O superconductors

Abstract

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 30.8 Å (2212 phase), 36.7 Å (2223 phase), and 43.2 Å (2234 phase) containing no intergrowth defects. The existence of two separate incommensurate superstructures, Q₁ and Q₂, which were previously observed in Pb-doped ceramics and single crystals, has been confirmed in this study. It is found that superlattice reflections from the Q₂,-modulated structure in the 2223 phase are much weaker than those from the Q₂-modulation, whereas the diffraction intensities of the Q₁- and Q₂-modulations in the 2212 phase are similar. We also show that the periodicity along the b-axis of the Q₂-modulation increases with increasing Pb-substitution, contrary to the implications of a previously suggested structural model.