Study of multivacancies in alpha Fe

Formation and migrations energies were calculated for multi-vacancies V-n (n = 1-15) in bcc Fe bulk and its surfaces using density functional theory. In this work, we analyze the evolution of the formation energy, the migration processes and the stable configurations of V-n. For small clusters (mono- and divacancies), our results are in agreement with those from the literature. We also focused on the study of the interactions between surfaces (Fe(1 0 0) and Fe(1 1 0)) and a vacancy, which enabled an assessment of the interactions the between vacancy and free surfaces. For V-3, we found that the more compact structure (labeled (1 1 2)) is a more stable configuration than that of V-4 (1 1 1 12 2). The migration mechanisms of V-2 and V-3 are decomposed to identify the position of the transition states, and to thus explain the low migration energy of V-3. An analysis of the structure of different clusters shows that for n <= 10, the more stable structures are built from 2NN and 1NN divacancies. We suggest some stable compact configurations for large clusters. (C) 2013 Elsevier B.V. All rights reserved.