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The microstructure of Sr-free bismuth cuprates, i.e., ${\mathrm{Bi}}_{2}$(${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{1+\mathit{x}}$)${\mathrm{CuO}}_{6.5+\mathrm{\ensuremath{\delta}}}$ and (${\mathrm{Bi}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$)${\mathrm{LaCaCuO}}_{6.5+\mathrm{\ensuremath{\delta}}}$, was determined by means of x-ray diffraction and electron diffraction (ED). ED analysis revealed that the two types of Sr-free compounds possess a monoclinic superstructural modulation, which is similar to ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{6+\mathrm{\ensuremath{\delta}}}$. In the ${\mathrm{Bi}}_{2}$(${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{1+\mathit{x}}$)${\mathrm{CuO}}_{6.5+\mathrm{\ensuremath{\delta}}}$ system, the modulation wavelength along the b and c axes increased markedly with the Ca substitution for La. At the same time this substitution facilitated the commensurate-to-incommensurate transition. For the (${\mathrm{Bi}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$)${\mathrm{LaCaCuO}}_{6.5+\mathrm{\ensuremath{\delta}}}$ system, a similar change in the modulation wave vector was also observed. The dependence of the modulation vector on chemical composition can be best interpreted by the crystal-misfit model. In addition, Raman scattering analysis of the ${\mathrm{Bi}}_{2}$(${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{1+\mathit{x}}$)${\mathrm{CuO}}_{6.5+\mathrm{\ensuremath{\delta}}}$ system showed that the vibrational properties of the oxygen atoms in the Bi-O bonding also changed with the increase in the modulation wavelength, and this behavior can be regarded as a consequence of the structural relaxation caused by the enhancement of the degree of crystal fit.

Raman measurements were performed on single-phase samples of Bi 2.1 Sr 1.9-x La x CuO y (0≤x≤1.0) and Bi 2-x Pb x Sr 2 CuO y (0.05≤x≤0.6). Significant modifications of the spectra, becoming more pronounced with increasing x, are noted with respect to the undoped Bi 2 Sr 2 CuO y in the energy ranges of 450-465 cm -1 and 610-650 cm -1 . The experimental results provide important data related to the origin of the incommensurate modulation structure and its relation to the superconductivity of the Bi 2 Sr 2 CuO y system