Formation of the High-Tc Phase in the Bi-Pb-Sr-Ca-Cu-O System in Various Atmospheres

Samples with compositions of Bi:Pb:(Sr, Ba):Ca:Cu=0.9:0.2:(1.0-x, x):1.0:1.6, in which O≦x≦0.07, were prepared by heating at 844°C in air after heating at 828°C in air or at 787°C in Ar. The Pb-free(or poor)low-Tc phase with a tetragonal structure was formed by heating at 828°C in air, while the heat treatment at 787°C in Ar led to formation of the Pb-rich low-Tc phase with anorthorhombic structure. The lattice parameter c of these low-Tc phases increases as x increases, resulting from the Ba substitution for Sr. By the final heating in air, the high-Tc phase forms through the Pb-rich low-Tc phase as well as the Pb-poor one. Ba substitution promotes the formation of the high-Tc phase and elongates its c-axis le.