Energy landscape of small clusters of self-interstitial dumbbells in iron

The activation-relaxation technique nouveau (ARTn), a method for the systematic search of the minima and saddle-point configurations, is applied to the study of interstitial-cluster defects in iron. Some simple modifications to improve the efficiency of the ARTn method for these types of applications are proposed. The energy landscapes at 0 K of defect clusters with up to four self-interstitial atoms obtained using the Ackland-Mendelev potential for iron are presented. The efficiency of the method is demonstrated in the case of monointerstitials. The number of different bound configurations increases very rapidly with cluster size from di- to quadri-interstitials. All these clusters can be analyzed as assemblies of dumbbells mostly with 110 orientation. The lowest-energy configurations found with the present method and potential are made of parallel dumbbells. The mechanisms associated with the lowest saddle-point energies are analyzed. They include local rearrangements that do not contribute to long-range diffusion. The translation-rotation mechanism is confirmed for the migration of mono- and di-interstitials. For the tri-interstitial the migration is dominated by three mechanisms that do not involve the lowest-energy configuration. The migration of quadri-interstitials occurs by an on-site reorientation of the dumbbells in the 111 direction, followed by the conventional easy glide. Finally, the minimum energy paths are investigated for the transformation toward the lowest-energy configuration of two specific clusters, including a quadri-interstitial cluster with a ring configuration, which was shown to exhibit an unexpected low mobility in previous molecular-dynamics simulations.