Stability and Mobility of Mono- and Di-Interstitials in $\alpha$-Fe

We report a detailed ab initio study of the stability and migration of self-interstitial atoms (SIAs) and di-interstitials (di-SIAs) in $\alpha$-Fe. The $⟨110⟩$ dumbbell is confirmed to be the most stable SIA configuration, 0.7 eV below the $⟨111⟩$ dumbbell. The lowest-energy migration path corresponds to a nearest-neighbor translation-rotation jump with a barrier of 0.34 eV. The most stable configuration for di-SIAs consists of $⟨110⟩$ parallel dumbbells. Their migration mechanism is similar to that for SIAs, with an activation energy of 0.42 eV. These results are at variance with predictions from existing empirical potentials and allow one to reconcile theory with experiments.

Figure 1

SIA formation energies in $\alpha$-Fe: comparison of the present SIESTA ab initio results for two supercell sizes with plane-wave (VASP) [8] and empirical potential (FS and MEAM) [21, 22] calculations. The crowdion (not represented) is always nearly degenerate with the $⟨111⟩$ dumbbell.

Figure 2

Schematic representation of the translation and/or rotation jumps of SIAs studied here with their respective activation energies: (a) translation rotation to first neighbor; (b) translation to first neighbor; (c) jump to second neighbor; (d) jump to third neighbor; (e) 60° on-site rotation; (f) $[110]$-to-$[111]$ on-site rotation. White and black spheres indicate initial and final positions of SIAs, their jumps are indicated by arrows in (a).

Figure 3

Energetic barrier along the migration pathways for SIAs (nearest-neighbor translation-rotation mechanism) and di-SIAs (one- and two-step mechanisms).

Figure 4

Schematic representation of the initial, (intermediate), and final configurations for the migration of di-SIAs by simultaneous (I) or successive (II) SIA jumps investigated here.