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Oxygen nonstoichiometry and valence of copper in the Cu-1222 superconductor |
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| Authors: | M Karppinen M Arai TS Chan JM Chen H Yamauchi |
| Institution: | a Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan b National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan, ROC c Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC |
| Abstract: | Essentially single-phase Cu(Ba0.67Eu0.33)2(Ce0.33Eu0.67)2Cu2O9±δ samples of the Cu-1222 phase with a fluorite-structured (Ce,Eu)-O2-(Ce,Eu) block between the superconductive CuO2 planes were synthesized in O2 atmosphere to exhibit superconductivity with Tc 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 N2 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 Tc 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. |
| Keywords: | Cu valence XANES spectroscopy |
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