Abstract
Essentially single-phase Cu(Ba 0.67Eu 0.33) 2(Ce 0.33Eu 0.67) 2Cu 2O 9± δ samples of the Cu-1222 phase with a fluorite-structured (Ce,Eu)–O 2–(Ce,Eu) block between the superconductive CuO 2 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 Ag 2O 2 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 CuO 2 plane and CuO 1± δ charge reservoir separately) in the samples is discussed on the bases of Cu L-edge and O K-edge XANES data.
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