Lead dioxide has been used for over a century in the lead-acid battery. Many fundamental questions concerning PbO(2) remain unanswered, principally: (i) is the bulk material a metal or a semiconductor, and (ii) what is the source of the high levels of conductivity? We calculate the electronic structure and defect physics of PbO(2), using a hybrid density functional, and show that it is an n-type semiconductor with a small indirect band gap of similar to 0.2 eV. The origin of electron carriers in the undoped material is found to be oxygen vacancies, which forms a donor state resonant in the conduction band. A dipole-forbidden band gap combined with a large carrier induced Moss-Burstein shift results in a large effective optical band gap. The model is supported by neutron diffraction, which reveals that the oxygen sublattice is only 98.4% occupied, thus confirming oxygen substoichiometry as the electron source.
Scanlon, D. O., Kehoe, A. B., Watson, G. W., Jones, M. O., David, W. I. F., Payne, D. J., Egdell, R. G., Edwards, P. P., & Walsh, A. (2011). Nature of the band gap and origin of the conductivity of PbO(2) revealed by theory and experiment. Physical Review Letters, 107(24), . https://doi.org/10.1103/PhysRevLett.107.246402