Oxygen and copper valencies in oxygen-doped superconducting La/sub 2/CuO/sub 4. 13/

Oxygen enrichment of La{sub 2}CuO{sub 4} by a high-temperature and high-presusre (1--3 kbar at 875 K) treatment produces a stable, bulk superconductor, La{sub 2}CuO{sub 4.13} (T{sub c}{approx}30 K). Annealing in vacuum above 350 K depletes the excess oxygen from La{sub 2}Cu{sub 4.13} as O{sub 2}(g), eliminates the superconductivity, and results in semiconducting La{sub 2}CuO{sub 4}. These annealed samples were subsequently recharged with oxygen, thereby reversibly restoring the bulk superconductivity. X-ray photoelectron spectroscopy has been used to identify the oxygen and copper valencies which correlate with the bulk superconductivity in this interstitially oxygen-doped perovskite material. An XPS O(1s) binding energy of 532.1 eV is measured for the excess oxygen which, with additional analytical and quantitative magnetization data, indicates that the predominant excess oxygen species is a superoxide ion (O{sub 2}). The angular dependence of the O(1s) peak intensity indicates a near-surface concentration enrichment of the superoxide species. The majority copper valency is Cu(II) with a Cu(2p) binding energy of 933.0{plus minus}0.1 eV; however, evidence for a minority Cu(III) species with a Cu(2p) binding energy of 934.4{plus minus}0.2 eV is found only in the superconducting, oxygen-enriched material.