Abstract
Well-defined and pure transitions at 108 and 75 K were obtained by partial substitution of Pb for Bi in the Bi-Sr-Ca-Cu-O system (nominal compositions of Bi 1.5Pb 0.5Sr 1.5Ca 2.5Cu 3O x and Bi 1.5Pb 0.5Sr 2.5Ca 1.5Cu 3O x, respectively). Fully-reversible effects on the superconducting properties of these phases as well as on the undoped 2:2:1:2 ( Tc = 85 K) composition were observed after thermal cycling under reducing and oxidizing atmospheres. It is shown, in particular, that by appropriate heat treatments it is possible to increase systematically the Tc of Bi 1.5Pb 0.5Sr 2.5Ca 1.5Cu 3O x by as much as 20 K. Opposite effects were observed for the high-Tc and low-Tc phases, and related to the initial content of hole carriers in these phases.
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