PROPERTIES OF POLYCRYSTALLINE ND1.85CE0.15CUO4-Y PREPARED UNDER DIFFERENT CONDITIONS

Polycrystalline samples of the electron-doped high-T-c superconductor Nd1.85Ce0.15CuO4-gamma were prepared with different starting materials , sintering temperatures, reducing atmospheres, and cooling rates afte r reduction. Observations of the microstructure and measurements of X- ray diffraction, electrical resistivity, and magnetic susceptibility i ndicate the important effects that cationic (Ce) and atomic (O) distri butions have upon the properties of this compound. A double resistive superconducting transition, attributed to small superconducting island s within grains and coupling between these islands, is an intrinsic pr operty of this compound. Samples sintered below the eutectic temperatu re are composed of small grains, are porous, and have an additional ph ase. An inhomogeneous cationic distribution results, even when an inte rmediate oxide NdCeO3.5 is used as a starring material in order to pro mote the diffusion of Ce into the matrix, as indicated by a large and semiconducting electrical resistivity, a broad resistive transition, a nd a small diamagnetic susceptibility For these samples, the oxygen di stribution has a pronounced effect on the resistive transition, with a more non-equilibrium distribution resulting in higher transition temp eratures. On the other hand, samples sintered above the eutectic tempe rature, and therefore with a liquid phase present, have larger grains and are dense. Their properties are relatively insensitive to the oxyg en distribution, which is inhomogeneous across the entire sample, whil e the Ce distribution is more uniform, as indicated by a small and nea rly metallic electrical resistivity, a sharper resistive transition, a nd a larger diamagnetic susceptibility. In addition, those samples pre pared with the intermediate oxide have higher transition temperatures. implications of the granularity of electron-doped compounds on the sm all diamagnetic contribution and also the absence of the peak in the s pecific heat at the superconducting transition are briefly discussed.