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 ${\mathrm{Bi}}_2$ ${\mathrm{O}}_3$ with the simultaneous development of the ${\mathrm{O}}^{2\mathrm{-}}$(2p) band (3 to 7 eV). We show that the 6s band of metals lies below the ${\mathrm{O}}^{2\mathrm{-}}$(2p) band, and hence 6s electrons cannot be ionized to form ${\mathrm{Pb}}^{4+}$ and ${\mathrm{Bi}}^{4+}$ as expected in ${\mathrm{PbO}}_2$, ${\mathrm{BaPbO}}_3$, ${\mathrm{BaBiO}}_3$, and ${\mathrm{BaPb}}_{0.75}$ ${\mathrm{Bi}}_{0.25}$ ${\mathrm{O}}_3$. Instead these oxides are stabilized with lower valent ${\mathrm{O}}_2^{2\mathrm{-}}$ along with ${\mathrm{O}}^{2\mathrm{-}}$ ions with metals remaining in +2 and +3 states. Similarly, it is shown that the ${\mathrm{Cu}}^{2+}$(3d) band overlaps with the ${\mathrm{O}}^{2\mathrm{-}}$(2p) band in the ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.95}$ completely and the excess oxygen can be stabilized through lower valent oxide ions instead of ${\mathrm{Cu}}^{3+}$. .AE