Evidence for double valence fluctuation in metallic oxides of lead

The possible role of double valence fluctuation in both lead and oxide ions with reference to metallization in oxides of lead is examined by x-ray-photoemission spectroscopy, ultraviolet-photoemission spectroscopy (UPS), and ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ NMR studies. The double valence fluctuations may be viewed as ${\mathrm{Pb}}^{4+}$+${2\mathrm{O}}^{2\mathrm{-}}$⇄${\mathrm{Pb}}^{2+}$ ${\mathrm{O}}_2^{2\mathrm{-}}$. While the insulating oxides PbO, ${\mathrm{Pb}}_3$ ${\mathrm{O}}_4$, and ${\mathrm{Sr}}_2$ ${\mathrm{PbO}}_4$ show a single oxide ion, ${\mathrm{O}}^{2\mathrm{-}}$ characterized by O(1s) at 529.7 eV, the insulating peroxide ${\mathrm{BaO}}_2$ is characterized by the ion ${\mathrm{O}}_2^{2\mathrm{-}}$ with a single O(1s) at 533 eV. The metallic ${\mathrm{PbO}}_2$, ${\mathrm{BaPbO}}_3$, ${\mathrm{BaBiPbO}}_3$, and ${\mathrm{SrPbO}}_3$ showed the occurrence of both ${\mathrm{O}}^{2\mathrm{-}}$ and ${\mathrm{O}}_2^{2\mathrm{-}}$ ions. The valence band in these compounds has also been studied by UPS, and clear evidence for the coexistence of ${\mathrm{O}}^{2\mathrm{-}}$ and ${\mathrm{O}}_2^{2\mathrm{-}}$ is seen in ${\mathrm{PbO}}_2$. A simultaneous study of ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$ NMR suggests that the Pb ion could also exist in mixed-valence states. Qualitative arguments are presented to rationalize the existence of such mixed valences of the anion in metal oxides in general and their role in superconductivity.