Lack of Cu³⁺, Pb⁴⁺, and Bi⁵⁺ ions in metallic and superconducting oxides

Abstract

With a view to understand the oxidation states of metal ions involved in oxide superconductors, oxidation behavior of Cu, Pb, and Bi metals have been studiedi employing x-ray-photoemission and ultraviolet-photoemission spectroscopy. Pb and Bi have distinct 6p (0 to 4 eV) and 6s (7.5 to 10 eV) bands and upon oxidation, only the 6p electrons are ionized forming PbO and Bi₂O₃ with the simultaneous development of the O^2-(2p) band (3 to 7 eV). We show that the 6s band of metals lies below the O^2-(2p) band, and hence 6s electrons cannot be ionized to form Pb⁴⁺ and Bi⁴⁺ as expected in PbO₂, BaPbO₃, BaBiO₃, and BaPb_0.75Bi_0.25O₃. Instead these oxides are stabilized with lower valent O₂^2- along with O^2- ions with metals remaining in +2 and +3 states. Similarly, it is shown that the Cu²⁺(3d) band overlaps with the O^2-(2p) band in the YBa₂Cu₃O_6.95 completely and the excess oxygen can be stabilized through lower valent oxide ions instead of Cu³⁺ AE.