PROPERTIES OF POLYCRYSTALLINE (ND1-XGDX)(1.85)CE0.15CUO4-Y COMPOUNDS

We have studied the structural, magnetic, and transport properties of polycrystalline (Nd1-xGdx)(1.85)Ce0.15CuO4-y; x=0.0, 0.05, 0.10, 0.25, and 0.50; produced under different atmospheres and cooling rates afte r the reduction process. From x-ray analysis the T'-structure is prese rved for all samples studied. We also found that quenched samples in a rgon atmosphere exhibit superconductivity below similar to 22 K. The t emperature dependence of the electrical resistivity rho(T) displays a striking double resistive superconducting transition with no evidence of a zero resistance state down to 1.5 K. The electrical resistivity h as a approximate to 20% drop at the upper critical temperature T-ci fo llowed by a plateau down to a lower temperature T-cj. While T-ci is bo th weakly dependent on the Gd content and external applied magnetic fi eld, the behavior of rho(T) at low temperatures is strongly sensitive to the Gd concentration, excitation current, and applied magnetic fiel ds. Also, the behavior of rho(T) below approximate to 3 K is almost fi eld independent. The magnetic susceptibility data chi(T) reveal superc onductivity below T-ci and a reentrantlike behavior at low temperature s where the magnitude of chi(T) increases rapidly with decreasing temp erature. These observations are similar to those observed in granular Ga films close to the nonmetallic side of the superconductor-insulator transition. We argue that these compounds can be understood within th e framework of a superconducting granular model. (C) 1997 American Ins titute of Physics.