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The variation of modulation in Y-doped Bi-based 2:2:1:2 phases has been studied by electron diffraction. The wave vector of the modulation can be written in the general form q=αa*+βb*+γc*, with β as the dominant component. Modulations with nonzero values of α have also been observed. While the value of β appears to be dependent on the Y-doping content, γ, which is unity for orthorhombic 2:2:1:2 phases, varies for a given value of β. The deviation of γ from unity breaks the orthorhombic symmetry (Bbmb), giving rise to monoclinic symmetry (B2/b or Bb). The variation of the wavelength of modulation with respect to Y-doping content and in situ heating in the electron microscope gives support to the model of extra oxygen in the BiO double layers as the origin of the incommensurate modulation.

Modulated structures and intergrowths in superconducting phases of nominal (Bi1−xPbx)2Sr2Ca2Cu3O10 (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 NBbmb111 (no. 66a), PBbml111 (no. 66a). The space group of the average structure of the two-dimensional modulated structure is Bbmb, and its Bravais class is CCmmm1p112 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.