Pb addition has been found to improve the superconducting properties of a Bi-Ca-Sr-Cu-O superconductor. We report zero resistance at 104 K in a ${\mathrm{Bi}}_{1.4}{\mathrm{Pb}}_{0.6}{\mathrm{Ca}}_2{\mathrm{Sr}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ alloy with no steps in the resistivity. Pb replaces Bi in the unit cell, as determined by x-ray microanalysis. The superconducting phase has the same structure as the alloy without lead, with the $c$ parameter uniformly 38.2 Å. The Cu+Ca content of these regions has also been found to be higher than in the undoped sample, indicating that the periodicity is determined by the composition leading to polytypoids. Such polytypoid structures appear to correlate well with the critical temperatures.

• Received 10 May 1988

DOI:https://doi.org/10.1103/PhysRevB.38.7070