Excess conductivity and diamagnetism in superconducting perovskitelike systems - Prospects for the increase of the critical temperature

The electric and magnetic properties of samples with compositions R-Ba-Cu-O (where R is a rare element or Y), Bi-Sr-Ca-Cu-O, and Tl-Ba-Ca-Cu-O were investigated at temperatures including the superconductivity region. It was found that, for most of the superconducting samples examined, the electric resistance, rho(T), and the magnetic susceptibility, chi(T), displayed a change in the character of temperature-dependence curves upon lowering the temperature below 180 K. It is shown that these changes may be explained by the presence in these 'single-phase' samples of one or more impurity phases with Tc values higher than those of the basic phase. The presence of impurity phases was confirmed by X-ray data.