XANES study of superconducting Lu0.7Ca0.3Ba2Cu3O7-alpha annealed in argon,air and oxygen

Cu K-XANES mesurements have been made on argon, air and oxygen annealed sup erconducting Lu0.7Ca0.3Ba2Cu3O7-delta compounds. The Cu1+ peak which is abs ent for the oxygen annealed sample, appears on air annealing and becomes pr ominent on argon annealing. For the argon annealed sample, the white line, assigned to Cu2+ shifts towards the lower energy side and its width increas es compared to that of the oxygen and the air annealed samples. From the ra tio 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 i ons 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. I n the light of the observed T-c values, the results are interpreted in term s of under-, near optimum-and over-doping of holes in the CuO2 planes respe ctively for the argon, air and oxygen annealing conditions. The analysis of the results provides convincing evidence for hole doping of the CuO2 plane s directly by the divalent Ca in LuBa2Cu3O7-delta.