Influence of chemical substituents on the superconductivity of the system Bi₄Ca₃Sr₃Cu₄O_y

The effect of the partial replacement of Bi by Eu, Gd, K and Pb, and Cu by Ni, on the superconducting transition temperature (T_c) of Bi ₄Ca₃Sr₃Cu₄O_y (y=undetermined) is investigated by DC resistivity (R) and AC susceptibility ( chi ) measurements. T_c is not significantly altered by Eu and Gd substituents (T_c close to 65 K); however, K and Pb substituents tend to enhance the fraction of a phase superconducting around 110 K. Interestingly, for nickel specimens, T_c of a large fraction, as suggested by AC chi data, initially decreases to 40 K with increasing concentration of nickel, followed by an increase, whereas R data shows no appreciable variation of T _c.