COMPLEX INFLUENCE OF THE BI-O RESERVOIR AND OXYGEN NONSTOICHIOMETRY ON THE HOLE DENSITY IN BI2SR2CACU2O8

X-ray-absorption near-edge spectra have been recorded at the Bi L3 and Cu L3 edges for the argon-synthesized Bi2Sr2CaCu2O8 (delta=0) phase and the oxygen-intercalated phases (delta>0) obtained by annealing the former phase under air and high-pressure oxygen. With respect to the reference Bi2O3 [Bi(III) valence state], edge shifts toward low energies have been found in the spectra of all the superconducting phases, indicating that the Bi formal valence is lower than III. This result confirms the role of hole reservoir of the [BiO]infinity layers. However, the shift is not constant but decreases with increasing oxygen intercalation and thus the contribution of the Bi-O reservoir to the hole density diminishes as the contribution of intercalated oxygen increases. Consequently, there is neither identity nor linear relationship between the oxygen content determined by thermogravimetric analysis and the hole density deduced from the simulations of the Cu L3 edges with two contributions due to covalent holes and doping holes.