Vacancies and vacancy-mediated self diffusion in Cr₂O₃: A first-principles study

Charged and neutral vacancies and vacancy-mediated self-diffusion in ?-Cr₂O₃ were investigated using first-principles density functional theory (DFT) and periodic supercell formalism. The vacancy formation energies of charged defects were calculated using the electrostatic finite-size corrections to account for electrostatic interactions between supercells and the corrections for the bandgap underestimation in DFT. Calculations predict that neutral oxygen (O) vacancies are predominant in chromium (Cr)-rich conditions and Cr vacancies with ?2 charge state are the dominant defects in O-rich conditions. The charge-transition levels of both O and Cr vacancies are deep within the bandgap, indicating the stability of these defects. Transport calculations indicate that vacancy-mediated diffusion along the basal plane has lower energy barriers for both O and Cr ions. The most favorable vacancy-mediated self-diffusion processes corresponds to the diffusion of Cr ion in Cr³⁺ charge state and O ion in O^2? state, respectively. Our calculations reveal that Cr triple defects composed of Cr in octahedral interstitial sites with two adjacent Cr vacancies along the c axis have a lower formation energy compared with that of charged Cr vacancies. The formation of such triple defects facilitates Cr self-diffusion along the c axis.