Effect of rare-earth Sm³⁺ substitution on the superconducting properties of the Bi_1.7Pb_0.3Sr₂Ca_2-xSm_xCu₃O_y system

The superconducting properties of the Bi_1.7Pb_0.3Sr₂Ca_2-xSm_xCu₃O_y system have been investigated by X-ray diffraction, electrical resistance, AC susceptibility and DC magnetization. Substitution of diluted quantities of Sm³⁺ for Ca²⁺ is found to change drastically the superconducting properties of the system, despite the weak paramagnetic nature of the Sm³⁺ ion. X-ray diffraction results show that the volume fraction of the high-T_c (2:2:2:3) phase decreases and that of the low-T_c (2:2:1:2) phase increases as the samarium concentration increases from x=0 to 0.05. At higher samarium concentrations, i.e. for x=0.075 and 0.1, only the low-T_c phase exists. AC susceptibility measurements (χ′ versus T) show that T_{c onset} does not change up to x=0.02, and then decreases down to 100 K for x=0.1. The loss peak (χ″ versus T) decreases from 108 to 104 K for x=0 to x=0.02, while for higher Sm concentrations, the χ″ peak decreases more significantly. The same trend was observed by resistivity measurements and can be explained on the basis of granular superconductivity. Magnetization measurements show that the superconducting volume (slope dM dH) decreases with increasing samarium concentration. These results show that the Ca²⁺→Sm³⁺ substitution changes the hole carrier concentration, which in turn lowers T_c and decreases the volume fraction of the superconducting phase.