Modulated structures and intergrowth in Pb-doped Bi-Sr-Ca-Cu-O superconducting oxides

Modulated structures and intergrowths in superconducting phases of nominal (Bi_1-x{Pb}ₓ)₂{Sr}₂Ca₂{Cu}₃O₁₀ (2:2:2:3) compounds have been studied by transmission electron microscopy. Three kinds of modulations have been observed, one is a two-dimensional modulation and the other two are one-dimensional modulations with superspace groups N 11{}1Bbmb (no. 66a), P_111Bbml (no. 66a). The space group of the average structure of the two-dimensional modulated structure is Bbmb, and its Bravais class is C₁ₚ₁₁₂Cmmm in a two-line symbol or Cₘₘₘ(1,0,{γ};0,0,{ν}) (no. 2-24) in a one-line symbol. It is found that phases with the same point group and similar structure (forming intergrowths) have the same modulation. Intergrowths with each of the three modulations have been observed. The next highest member of the structural series, the 2:2:3:4 phase, has been found as having intergrowths. On the basis of the modulations of the intergrowths being the same, the modulations in the 2:2:3:4 phase have been determined.