  First principles density functional theory (DFT) investigation of native
interstitials and the associated self-diffusion mechanisms in {\alpha}-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 bandgap. 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.
