Conditions for superconductivity in the electron-doped copper-oxide system, (Nd1-xCex)2CuO4+δ

We report systematic studies on the relations among the Ce-IV-for-Nd-III substitution level (x), oxygen-partial pressure (P-O2), oxygen content (4 + delta), lattice parameters (a, c) and superconductivity characteristics (T-c, volume fraction) in the (Nd1-xCex)(2)Cu1-yO4+delta system which includes electron-doped superconductors. Independent of the Ce-doping level x, samples synthesized in air are found oxygen deficient, i.e. delta < 0. Nevertheless, reductive annealing is needed to induce superconductivity in the air-synthesized samples. At the same time, the arnount of oxygen removed upon the annealing is found very small (e.g. 0.004 oxygen atoms per formula unit at x = 0.075), and consequently the effect of the annealing on the valence of copper (and thereby also on the electron doping level) is insignificant. Rather, the main function of the reductive annealing is likely to repair the Cu vacancies believed to exist in tiny concentrations (y) in the air-synthesized samples. (c) 2007 Elsevier Inc. All rights reserved.