Crystallochemical characterization of Ce/RE substituted phases of T′ structure type: Quantitative analysis of Cu valence ( V ) and its content (1− y ) in superconducting sample of nominal composition Nd 1.95−x Ce x CuO 4 ( x =0.15)

New explanation is given concerning phase separation i.e., consequences of Cu(1 - y) deficiency, generally observed in samples of nominal stoichiometry RE2-xCexCuO4 (RE = lanthanide), reported to happen in T' structure phases during synthesis in air above 1000 degrees C. Contrary to some literature data, it is assumed herein that phase separation mentioned takes place not during reductive annealing but already during synthesis in air. Accompanying evaporation of Cu, this process leads to RE-site deficient T' skeleton(s). Based on literature available phase equilibria in Nd2O3-CeO2-CuO system, plausible structure deficiencies are presented and discussed. It is shown that reductive annealing of so defected T' matrices leads to removal of only tiny amounts of oxygen, the latter resulting from higher than +2 Cu valence and from Cu deficiency factor (1 - y). As confirmed on Nd1.95-xCexCuO4 sample (x = 0.15), doping of Cu ions with electrons appears not to be a crucial factor for appearance of superconductivity in T' phase. (C) 2012 Elsevier B.V. All rights reserved.