XANES study of superconducting Lu0.7Ca0.3Ba2Cu3O7-alpha annealed in argon, air and oxygen
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Cu K-XANES mesurements have been made on argon, air and oxygen annealed superconducting Lu0.7Ca0.3Ba2Cu3O7-delta compounds. The Cu1+ peak which is absent for the oxygen annealed sample, appears on air annealing and becomes prominent on argon annealing. For the argon annealed sample, the white line, assigned to Cu2+ shifts towards the lower energy side and its width increases compared to that of the oxygen and the air annealed samples. From the ratio of areas of Cu1+ and Cu2+ deconvoluted peaks and the oxygen content (O7-delta), it has been possible to estimate the effective charges (n) on Cu ions separately for the Cu(1) chain and the Cu(2) plane sites. The value of n > 2 on Cu of the Cu(2) sheets is found essential for superconductivity. In the light of the observed T-c values, the results are interpreted in terms of under-, near optimum-and over-doping of holes in the CuO2 planes respectively for the argon, air and oxygen annealing conditions. The analysis of the results provides convincing evidence for hole doping of the CuO2 planes directly by the divalent Ca in LuBa2Cu3O7-delta.
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