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XPS study of the dependence on stoichiometry and interaction with water of copper and oxygen valence states in the YBa2Cu3O7−x compound

1989; Elsevier BV; Volume: 81; Issue: 2 Linguagem: Inglês

10.1016/0022-4596(89)90011-x

1095-726X

Pedro Salvador, J.L.G. Fierro, Juan Antonio Amador, C. Cascales, I. Rasines,

Advanced Condensed Matter Physics

In order to elucidate the valence states of both copper and oxygen in YBa2Cu3O7−x as a function of the oxygen content, their O1s and Cu2p core-level X-ray photoelectron spectra were studied at room temperature for 0.9 ≥ x ≥ 0.1. No evidence of the Cu3+ (i.e., 3d8) 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 3d9KL (i.e., Cu2+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 3d10 (i.e., Cu+). The amount of monovalent copper was found to increase with x, as oxygen (O− species) from Cu1O chains parallel to the crystallographic b axis leave the lattice and electrons are transferred to the adjacent Cu2+ 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 Cu2+. The high reactivity of the superconducting material with water is evidenced by the special characteristics of its O1s 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 O2− 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 O2− signal could hardly be observed. These results are consistent with the existence of delocalized holes in O2p orbitals. In fact, the O− lattice species strongly react with water molecules to produce more stable OH0 radicals, which further recombine to generate H2O2 whose decomposition is catalyzed by Cu2+ 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 Ba2+ ions as inferred from the analysis of the Ba3d XPS spectrum.