First-Principles Investigation of Native Interstitial Diffusion in Cr₂O₃

Abstract

First principles density functional theory (DFT) investigation of native interstitials and the associated self-diffusion mechanisms in α-Cr 2 O 3 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.

Temperature Dependence of Self-Diffusion in Cr2O3 from First Principles

“…The resulting diffusion coefficients have a reasonable agreement with the experimental data. This work can be considered as the culmination of our previous studies on the atomistic diffusion mechanisms in Cr 2 O 3 mediated by vacancies and interstitials [23,28]. For barrier energies, the transition state and ground defect state exhibit similar local electronic environment, suggesting an insensitivity to the U value used in this prior work.…”

“…When compared to the corrected transition levels reported in our previous works, the corresponding transition levels obtained with HSE are overall at a higher Fermi level. Some of the transition levels such as Cr i (3/2) transition, which is at 2.6 eV above the VBM, differ by nearly 1 eV (1.64 eV in Ref [28]). Further, HSE predicts that only +3 and +2 charge states are stable for Cr i within the bandgap.…”

“…where p and n represent the number densities of holes and electrons, respectively. Given that the formation energy of a charged defect can be calculated as [23,28]…”

“…For bandgap fitting, we used a fixed cell size that was optimized with PBEsol+U approach in our previous studies. [23,28] The bandgaps obtained with different screening parameters are listed in Table 1. The results indicate that calculations with the screening parameter of 0.6 provide the best match between the computed and experimental bandgap.…”

“…Complementing the experimental investigations, diffusion mediated by both intrinsic and extrinsic point defects in Cr 2 O 3 have been extensively studied theoretically by many groups, including ours [21,22,23,24,25,26,27,28]. Due to the advances in density functional theory (DFT) accuracy in calculating many properties of materials, recent studies employed DFT to evaluate formation energies, migration barriers and charge localization of point defects [22,23,24,28,27]. The 0K energies obtained from DFT were combined with Einstein-Smoluchowski random walk thermal diffusion formalism [29] to predict the self-diffusion coefficients in Cr 2 O 3 .…”