Preparation, formation kinetics, and properties of polycrystalline Nd_1.85Ce_0.15Cu_4-y obtained from a sol-gel precursor

Polycrystalline samples of Nd_1.85Ce_0.15CuO_4-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 °C, undergoes several decomposition steps up to about 500 °C, where the beginning of Nd_2-xCe_xCuO_4-y (x≈0) formation is observed. At 700 °C, the predominant phase is Nd_2-xCe_xCuO_4-y which coexists with CuO and CeO₂. Single-phase Nd_1.85Ce_0.15CuO_4-y is obtained by heat treating the precursor in air at 1000 °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.