XPS study of the dependence on stoichiometry and interaction with water of copper and oxygen valence states in the YBa 2Cu 3O 7−x compound

Abstract

In order to elucidate the valence states of both copper and oxygen in YBa 2Cu 3O 7− x as a function of the oxygen content, their O 1 s and Cu 2 p core-level X-ray photoelectron spectra were studied at room temperature for 0.9 ≥ x ≥ 0.1. No evidence of the Cu 3+ (i.e., 3 d 8) configuration was found for the ground state of a superconducting sample ( x ⋍ 0.1). Rather, the ground state for this composition can be described as a mixture of two configurations: mainly 3 d 9 KL (i.e., Cu 2+O − hybridization), where K represents an electron of the conduction band and L stands for a hole in the oxygen bonded to a virtually divalent copper, and some 3 d 10 (i.e., Cu +). The amount of monovalent copper was found to increase with x, as oxygen (O − species) from Cu 1O chains parallel to the crystallographic b axis leave the lattice and electrons are transferred to the adjacent Cu 2+ ions. Simultaneously, the concentration of holes delocalized in the oxygen valence band decreases, the Fermi level goes upward, and the material's behavior at room temperature changes from quasi-metallic (degenerated p-type semiconductor) to p-type semiconducting. For x ⋍ 0.9 the amount of Cu + predominates over that of Cu 2+. The high reactivity of the superconducting material with water is evidenced by the special characteristics of its O 1 s core-level spectrum. The presence of OH − ions indicates dissociative adsorption of water molecules from the air. The XPS signal due to OH − species is higher than that of the O 2− lattice ions, even when the sample was preserved from exposure to air. Moreover, when the superconducting sample was contaminated by prolonged exposition to air, the O 2− signal could hardly be observed. These results are consistent with the existence of delocalized holes in O 2 p orbitals. In fact, the O − lattice species strongly react with water molecules to produce more stable OH 0 radicals, which further recombine to generate H 2O 2 whose decomposition is catalyzed by Cu 2+ ions. As a result, molecular oxygen from lattice O − species is evolved. Charge neutrality in the superconductor lattice is preserved by filling O − vacancies with OH − groups, which became coordinated to Ba 2+ ions as inferred from the analysis of the Ba 3 d XPS spectrum.