Study of multivacancies in alpha Fe

Formation and migrations energies were calculated for multi-vacancies Vn (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 Vn. 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 V3, we found that the more compact structure (labeled (1 1 2)) is a more stable configuration than that of V4 (1 1 1 1 2 2). The migration mechanisms of V2 and V3 are decomposed to identify the position of the transition states, and to thus explain the low migration energy of V3. 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.