title: Nature of the band gap and origin of the conductivity of PbO2 revealed by theory and experiment.
abstract: Lead dioxide has been used for over a century in the lead-acid battery. Many fundamental questions concerning PbO2 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 PbO2, using a hybrid density functional, and show that it is an n-type semiconductor with a small indirect band gap of ∼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.
authors.affiliation: School of Chemistry and CRANN, Trinity College Dublin, Dublin 2, Ireland.
authors.forename: David O
authors.initials: DO
authors.lastname: Scanlon
authors.forename: Aoife B
authors.initials: AB
authors.lastname: Kehoe
authors.forename: Graeme W
authors.initials: GW
authors.lastname: Watson
authors.forename: Martin O
authors.initials: MO
authors.lastname: Jones
authors.forename: William I F
authors.initials: WI
authors.lastname: David
authors.forename: David J
authors.initials: DJ
authors.lastname: Payne
authors.forename: Russell G
authors.initials: RG
authors.lastname: Egdell
authors.forename: Peter P
authors.initials: PP
authors.lastname: Edwards
authors.forename: Aron
authors.initials: A
authors.lastname: Walsh