Superconducting phase transitions in the La-M-Cu-O layered perovskite system,M=La, Ba, Sr, and Pb

We report results of our studies of the La-Mu-Cu-O system, M=La, Ba, Sr, and Pb. $T_c$ and the magnetic moment are correlated with annealing schedules and x-ray structural analyses. The occurrence of superconductivity is associated with the formation of ${\mathrm{Cu}}^{3+}$ ions producing an expansion of the c parameter of the unit cell. We have found magnetic and resistive anomalies throughout near 65 K in some La-Ba-Cu-O samples. These samples show a slight depression of the superconducting transition temperature.