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 have raised many questions about the composition, off-stoichiometry, and copper doping itself, which are related to native defect chemistry. We perform first-principles defect calculations to provide much needed insights into the defect chemistry and doping of Pb₁₀(PO₄)₆O. We find that Fermi energy pinning in the midgap region occurs due to Pb and O vacancies. Our calculations also suggest the plausible existence of closely related off-stoichiometric phase(s); we predict one such phase. We predict only moderate levels of Cu doping, which calls into question the experimental claim of 10% incorporation on the Pb sites. Cu substitution on the Pb(1) and Pb(2) Wyckoff sites is possible, resulting in Cu d⁹ and d¹⁰ electronic configurations, respectively. We predict 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.