Properties of polycrystalline samples of Nd2−xCexCuO4−y obtained from a sol-gel precursor

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Abstract
Polycrystalline samples of Nd2−x Ce x CuO4−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 parameterc and a small increase in the lattice parametera are observed with increasing Ce concentration. Electrical resistanceR(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 resistivesuperconducting transition and that the zero resistance state is achieved through two distinct drops in R(T) at upper and lower transition temperatures T ci and T cj . 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 T cj 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 T cj . The results are discussed within the framework of a granular superconductor model.

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