Properties of polycrystalline Nd 1.85Ce 0.15CuO 4−γ prepared under different conditions

Abstract

Polycrystalline samples of the electron-doped high- T c superconductor Nd 1.85Ce 0.15CuO 4−γ were prepared with different starting materials, sintering temperatures, reducing atmospheres, and cooling rates after reduction. Observations of the microstructure and measurements of X-ray diffraction, electrical resistivity, and magnetic susceptibility indicate the important effects that cationic (Ce) and atomic (O) distributions have upon the properties of this compound. A double resistive superconducting transition, attributed to small superconducting islands within grains and coupling between these islands, is an intrinsic property of this compound. Samples sintered below the eutectic temperature are composed of small grains, are porous, and have an additional phase. An inhomogeneous cationic distribution results, even when an intermediate oxide NdCeO 3.5 is used as a starting material in order to promote the diffusion of Ce into the matrix, as indicated by a large and semiconducting electrical resistivity, a broad resistive transition, and a small diamagnetic susceptibility. For these samples, the oxygen distribution has a pronounced effect on the resistive transition, with a more non-equilibrium distribution resulting in higher transition temperatures. On the other hand, samples sintered above the eutectic temperature, and therefore with a liquid phase present, have larger grains and are dense. Their properties are relatively insensitive to the oxygen distribution, which is inhomogeneous across the entire sample, while the Ce distribution is more uniform, as indicated by a small and nearly metallic electrical resistivity, a sharper resistive transition, and a larger diamagnetic susceptibility. In addition, those samples prepared with the intermediate oxide have higher transition temperatures. Implications of the granularity of electron-doped compounds on the small diamagnetic contribution and also the absence of the peak in the specific heat at the superconducting transition are briefly discussed.