Defect Chemistry and Doping of Lead Phosphate Oxo Apatite Pb10(PO4)6O

Lead phosphate oxo apatite Pb10(PO4)6O is claimed to host room-temperature superconductivity when doped with copper. However, unsuccessful attempts to reproduce this claim has raised many questions about the composition, off-stoichiometry, and copper doping itself, among others. These questions are related to the native defect chemistry and thermodynamic phase stability. We perform first-principles defect calculations to provide much needed insights into the defect chemistry and doping of Pb10(PO4)6O. We find that the dominant Pb and O vacancies pin the Fermi energy in the mid-gap region. Our calculations suggest the plausible existence of closely related off-stoichiometric phase(s); we predict one such phase. We predict moderate levels of Cu doping, which still results in insulating behavior that is consistent with single-crystal measurements. We also rule out interstitial Cu doping, but find that unintentional S incorporation is highly possible. Our findings emphasize the need for careful characterization of the parent composition and the identification of synthesis conditions that will maximize (minimize) intentional (unintentional) doping.