Direct observation of atomic columns in a Bi-2223 polycrystal by aberration-corrected STEM using a low accelerating voltage

•We investigate a Ag-sheathed Bi-2223 wire by low-voltage aberration-corrected STEM.•Atomic displacements enable each layer to be continuous at edge grain boundaries.•The grains are terminated with deficient (Bi,Pb)–O layers at the other boundaries.•EELS mapping visualizes atomic columns in the intergrowth of Bi-2212 and 2234 phases.•HAADF analysis shows modulation of the occupancy of the (Bi,Pb) sites along [1 1 0].

Aberration correction in scanning transmission electron microscopy (STEM) enables an atomic-scale probe size of ∼0.1 nm at a low accelerating voltage of 80 kV that avoids knock-on damage in materials including light elements such as oxygen. We used this advanced method of microscopy to directly observe atomic columns in a (Bi,Pb)2Sr2Ca2Cu3O10+δ (Bi-2223) superconducting wire produced by a powder-in-tube method. Using the atomic-number (Z) contrast mechanism, incoherent high-angle annular dark-field (HAADF) imaging clearly showed the atomic columns. Atomic displacements toward the boundary with a maximum magnitude of ∼0.26 nm enable each atomic layer to be continuous at edge grain boundaries (EGBs). The grains tend to be terminated with deficient (Bi,Pb)–O single layers at c-axis twist boundaries (TWBs) and small-angle asymmetrical tilt boundaries (ATBs); a quantitative HAADF analysis showed that the occupancies of the (Bi,Pb) sites around these boundaries are ∼0.66 and ∼0.72, respectively. Electron energy-loss spectroscopy (EELS) mapping successfully visualized atomic columns in the half-unit cell intergrowth of (Bi,Pb)2Sr2CaCu2O8+δ (Bi-2212) and (Bi,Pb)2Sr2Ca3Cu4O12+δ (Bi-2234) phases. Furthermore, the HAADF analysis indicated that the occupancy of the (Bi,Pb) sites is modulated between ∼0.88 and 1.0 along the diagonal direction of the primitive perovskite cell with the same period as the structural modulation.