Superconductivity in rare-earth-doped oxygen-defect perovskites La_2-x-yLn_ySr_xCuO_4-z

The structural, magnetic, and electronic properties of the mixed compounds La_2-x-yLn_ySr_xCuO₄ for 0.15<x<0.25, 0.1<y<0.2 and Ln=Pr, Nd, Sm, Eu, Gd were studied. The magnetic susceptibility and the variation of the lattice parameters suggests all these rare earths are present in these materials as trivalent ions. DC resistance, ac susceptibility, Meissner and shielding measurements have shown that superconductivity persists among several members of the series. The magnetic rare earth ions depress T_c but do not destroy superconductivity. Within the series x=0.2 and y=0.3, the superconductivity transition temperatures, taken at the midpoint in resistivity, decrease from a maximum value at La (36.8K) to a minimum value at Gd (18.5K). It is suggested that the unit cell volume contraction observed through the lanthanide sequence, rather than magnetic interactions, is responsible for the observed depression in T_c.