Polarons, vacancies, vacancy associations, and defect states in multiferroic BiFeO3

Abstract

Oxygen, bismuth, and iron vacancies are studied by density-functional theory calculations in multiferroic bismuth ferrite in oxidative and reducing external conditions. We accurately describe the localized electronic states associated with isolated defects. Oxygen-rich conditions provide fully ionized oxygen vacancies, and ionized or partially ionized cationic vacancies, possibly yielding $p$-type conductivity mediated by oxygen-type holes. Oxygen-poor conditions provide fully ionized vacancies and a much smaller concentration of electronic carriers. Cationic and oxygen vacancies tend to pair (Bi-O, Fe-O) with an associated electric dipole being as close as possible to the polar axis, possibly leading to imprint effects. This clustering also results in an extended stability domain for the ionized defects. Furthermore, O and Fe vacancies are respectively associated with Fe-$3d$ and O-$2p$ type moderately deep defect states spatially localized close to the defect. The charge-neutral Fe vacancy possesses an electric dipole, despite being a point defect. Bi vacancies may be found in several metastable electronic states, with in the most stable one, a mid-gap band of defect states with Fe-$3d$ character, in addition to the three O-$2p$ defect states associated to the acceptor nature of this defect. Finally, electrons released in the lattice by oxygen vacancies, if not captured by acceptor defects, tend to localize as small polarons, while holes released by cationic vacancies, if not captured by donor defects, are associated with delocalized Bloch-like band states.