PREPARATION, FORMATION KINETICS, AND PROPERTIES OF POLYCRYSTALLINE ND1.85CE0.15CUO4-Y OBTAINED FROM A SOL-GEL PRECURSOR

Polycrystalline samples of Nd1.85Ce0.15CuO4-y obtained from a sol-gel precursor were sintered and reduced under different conditions and studied by high resolution X-ray powder diffraction, electrical resistivity, and magnetic susceptibility measurements. X-ray diffraction measurements showed that the precursor, which is non-crystalline at temperatures as high as 300-degrees-C, undergoes several decomposition steps up to about 500-degrees-C, where the beginning of Nd2-xCexCuO4-y (x almost-equal-to 0) formation is observed. At 700-degrees-C, the predominant phase is Nd2-xCexCuO4-y which coexists with CuO and CeO2. Single-phase Nd1.85Ce0.15CuO4-y is obtained by heat treating the precursor in air at 1000-degrees-C for 20 h. Electrical resistivity measurements on samples sintered below and above the eutectic temperature, and reduced under different conditions, reveal a semiconductor-like temperature dependence of the normal-state electrical resistivity. In addition, a striking double resistive superconducting transition with a Josephson-like lower temperature transition seems to be a common characteristic of these polycrystalline samples. Magnetic susceptibility measurements reveal a significant diamagnetic contribution at 5 K which occurs only below the Josephson coupling temperature. These features are discussed within the framework of a granular superconductor model.