X-ray photoelectron spectroscopic evidences for Cu3+ valence state in superconducting YBa2Cu3O6.91

X-ray photoelectron spectra of the Cu 2p and O 1s core levels of YBa2Cu3O7-δ(δ=0.09 and 0.40) provide the authors with the evidence of the mixed valency of Cu2+ and Cu3+ in YBa2Cu3O6.91. The binding energies of Cu(2p3/2 and 2p1/2 states) in YBa2Cu3O7-δ samples are compared with those of La2CuO4 and La4LiCuO8 which contain Cu2+ and Cu3+, exclusively. The spectra of Cu 2p electrons in the 90K superconducting YBa2Cu3O6.91 can be represented as overlapping spectra of the two lanthanum-cuprates. The Ols XPE spectra disclosed at least two, possibly three, with the major contribution at the binding energies of 529.6 and 532.0 eV. The authors have found no evidence for the existence of O22- species (Binding Energy ≥ 533 eV) in the prepared Y-Ba-Cu oxides, nearly contamination-free samples. Additionally, absence of peroxide ion (O22-) in YBa2Cu3O7-δ has been confirmed by colorimetric test using Ti4+ solution