First-Principles Investigation of Native Interstitial Diffusion in Cr₂O₃

First-principles density functional theory investigation of native interstitials and the associated self-diffusion mechanisms in α-Cr₂O₃ reveals that interstitials are more mobile than vacancies of corresponding species. Cr interstitials occupy the unoccupied Cr sublattice sites that are octahedrally coordinated by 6 O atoms, and O interstitials form a dumbbell configuration orientated along the [221] direction (diagonal) of the corundum lattice. Calculations predict that neutral O interstitials are predominant in O-rich conditions and Cr interstitials in +2 and +1 charge states are the dominant interstitial defects in Cr-rich conditions. Similar to that of the vacancies, the charge transition levels of both O and Cr interstitials are located deep within the band gap. Transport calculations reveal a rich variety of interstitial diffusion mechanisms that are species-, charge-, and orientation-dependent. Cr interstitials diffuse preferably along the diagonal of corundum lattice in a two-step process via an intermediate defect complex comprising a Cr interstitial and an adjacent Cr Frenkel defect in the neighboring Cr bilayer. This mechanism is similar to that of the vacancy-mediated Cr diffusion along the c-axis with intermediate Cr vacancy and Cr Frenkel defect combination. In contrast, O interstitials diffuse via bond switching mechanism. O interstitials in -1 and -2 charge states have very high mobility compared to neutral O interstitials.