PROPERTIES OF POLYCRYSTALLINE SAMPLES OF ND2-XCEXCUO4 FROM A SOL-GEL PRECURSOR

Polycrystalline samples of Nd2-xCexCuO 4-y(0.0≤x≤0.17) obtained from a sol-gel precursor were sintered below the eutectic temperature and reduced under different conditions. From the results of x-ray powder diffraction, the tetragonal T' structure is preserved for Ce content up to x=0.17. A decrease in the lattice parameter c and a small increase in the lattice parameter a are observed with increasing Ce concentration. Electrical resistance R(T) measurements performed on reduced samples with x≥0.12 reveal superconducting behavior below T≊25 K. We also found that all reduced samples display a striking double resistive superconducting transition and that the zero resistance state is achieved through two distinct drops in R(T) at upper and lower transition temperatures Tci and Tcj. The upper transition temperature T ci has a maximum for Ce concentration x=0.14, which is independent of the reduction process. The lower transition temperature Tcj is observed in reduced samples with Ce content x≥0.14 and decreases monotonically with increasing Ce concentration. Magnetic susceptibility measurements confirm bulk superconductivity in several reduced samples, and a significant diamagnetic contribution is only observed for temperatures below Tcj. The results are discussed within the framework of a granular superconductor model.