Oxygen nonstoichiometry and valence of copper in the Cu-1222 superconductor

Essentially single-phase Cu(Ba0.67Eu0.33)(2)(Ce0.33Eu0.67)(2)Cu2O9+δ samples of the Cu-1222 phase with a fluorite-structured (Ce,EU)-O-2-(Ce,Eu) block between the superconductive CuO2 planes were synthesized in O-2 atmosphere to exhibit superconductivity with T-c around 25 K. Wide-range tuning of oxygen content and thereby the overall hole-doping level of the phase was found possible through (i) temperature-controlled oxygen-depletion (TCOD) annealing carried out in a thermobalance in N-2 at various temperatures (for reduction), and (ii) high-pressure oxygenation (HPO) treatments carried out in a cubic-anvil-type high-pressure apparatus in the presence of various amounts of Ag2O2 as an excess oxygen source (for oxidation). For the HPO samples a record-high T-c value of 62 K was achieved. On the other hand, deoxygenation to the oxygen content less than 9 ± δ ≈ 8.9 was found to kill superconductivity. The degree of hole doping (both in overall and for the CuO2 plane and CuO1±δ charge reservoir separately) in the samples is discussed on the bases of Cu L-edge and O K-edge XANES data. © 2005 Elsevier Inc. All rights reserved.