Why is lead dioxide metallic?

The electronic structure of lead dioxide (PbO₂) has been studied by high resolution valence band X-ray photoelectron spectroscopy and electron energy loss spectroscopy, supported by band structure calculations carried out within the framework of density functional theory. The metallic nature of PbO₂ is shown to arise from occupation of conduction band states above the Fermi level of stoichiometric PbO₂, probably arising from oxygen vacancy defects. Strong satellites are observed in core level photoemission spectra at an energy consistent with the plasmon frequency observed in electron energy loss spectra.