Stability of superconducting phases in Bi-Sr-Ca-Cu-O and the role of Pb doping

The effects of heat treatment, partial oxygen pressure ), and Pb substitution on the oxygen nonstoichiometry and on the stability of superconducting phases in the Bi-Sr-Ca-Cu-O systems were examined. The Pb substitution promoted and stabilized the 110-K phase, with a reproducible zero resistance at 107 K hardly affects the superconducting transition temperature (Tc) in Pb stabilized samples while a large variation of Tc in Bi-Sr-Ca-Cu-O from 0.01 to 1.0 atm. The Cu³⁺ concentrations determined by a volumetric over the range studied, so this ion is not critical for superconductivity in Bi-Sr-Ca-Cu-O, as it is in Y-Ba-Cu-O. The higher extra oxygen content obtained from iodometric titration is attributed to the oxygen associated with Bi⁵⁺ ions in Bi-O₂ layers which are not measured by the volumetric method. Such ions result from cationic vacancies. In doped 2:2:2:3 compounds Pb⁴⁺ ions, most likely located in the Bi layers, lower the charge states of both Bi and Cu which are otherwise raised by cationic vacancies.