Stability of uncommon interstitial clusters in Fe

We present empirical potential and Density Functional Theory calculations on small interstitial clusters of unconventional shape in Fe. Empirical potential simulations of displacement cascades and subsequent annealing resulted in the formation of a number of interstitial clusters with nonstandard configurations which are characterized by non-parallel <110> dumbbells and di-interstitials consisting of three atoms sharing one lattice site. Such clusters were found to be essentially immobile as compared to the clusters of conventional shapes (i.e. platelets of <110> dumbbells or <110> crowdions) and be able to grow by absorbing other interstitials. Both empirical potential and Density Functional Theory calculations indicate that the formation energies of these alternative structures are not much higher than or as high as those of the conventional configurations. The interstitial clusters were observed to remain in these alternative configurations for substantial periods of time remaining immobile during empirical-potential molecular dynamics simulations and the process of transformation from an immobile state into a regular cluster was found to be thermally activated. Therefore the presence of such immobile states may have an impact on the interstitial cluster mobility. The energetics of found immobile clusters and transformation mechanisms between immobile and mobile states are discussed. (authors)