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

It is claimed that copper (Cu) doping of lead phosphate apatite Pb₁₀(PO₄)₆O causes volume contraction and, consequently, room-temperature superconductivity. Unsuccessful attempts to reproduce these claims have raised many questions about the reported composition, possible oxygen off-stoichiometry, and copper doping itself, among others. Additionally, unintentional incorporation of other elements (besides Cu) present in the initial reaction mixture has been speculated. Many of these questions are intimately 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 Pb and O vacancies are the dominant defects that pin the Fermi energy in the mid-gap region consistent with an insulating behavior. Our defect calculations suggest the plausible existence of closely related Pb and O sub-stoichiometric phases; we predict one such stable phase. We predict moderate levels of Cu incorporation on the Pb site, 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 more careful characterization of the parent composition, and intentional and unintentional dopant incorporation in the structure.