First Principles Study of Bismuth Vacancy Formation in (111)-Strained BiFeO₃

Epitaxial strain is known to significantly influence the structural and functional properties of oxide thin films. However, its impact on point defect concentration has been less explored. Due to the challenges in experimentally measuring thin-film stoichiometry, computational studies become crucial. In this work, we use first-principles calculations based on density functional theory to investigate the formation and stability of Bi vacancies and Bi-O vacancy pairs in BiFeO₃ (BFO) under (111) epitaxial strain. Our results demonstrate that compressive strain (-4%) decreases the formation enthalpy of Bi vacancies by 0.88 eV, whereas tensile strain (4%) increases it by 0.39 eV. Out-of-plane (OP) Bi-O vacancy pairs exhibit enhanced stability under both compressive and tensile strain, with formation enthalpy reductions of 1.49 eV and 1.05 eV, respectively. In contrast, in-plane (IP) vacancy pairs are stabilized under compressive strain but are insensitive to tensile strain. Finally, we discuss how these findings influence Bi stoichiometry during thin-film growth and the role of local strain fields in the formation of conducting domain walls.