Samples with nominal composition ${\mathrm{Bi}}_{1.8}$${\mathrm{Pb}}_{0.35}$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{y}}$ annealed in oxygen and nitrogen atmospheres at 500 and 750 °C are characterized by use of x-ray diffraction, magnetic measurements, oxygen-content determination, and electron diffraction. For samples annealed in an oxygen atmosphere, there exist two kinds of modulated structure: a Bi-type modulation and a Pb-type modulation. For samples annealed in ${\mathrm{N}}_2$ flow at 500 °C, Bi-type modulations disappear completely, although Pb-type modulations remain almost unchanged; however, for samples annealed in ${\mathrm{N}}_2$ flow at 750 °C, Pb-type modulations also disappear. It suggests that the Bi-type modulation originates from the extra oxygen in the Bi-O layers and the Pb-type modulation may arise not only from the excess oxygen but also from compositional modulations. ${\mathit{T}}_{\mathit{c}}$’s of these samples are found not to change significantly with annealing conditions and varying oxygen content, and it is concluded that the modulated structure is not crucial to the superconductivity.

• Received 23 December 1991

DOI:https://doi.org/10.1103/PhysRevB.46.9181