Vacancy clusters in graphite have been investigated using Density Functional Theory (DFT) within B3LYP exchange-functional. The smallest size of vacancy clusters (V4) has been chosen to study the migration energy and aggregation mechanism. Two main types of V4 vacancy clusters have been modeled, the disc ( V4) and the line vacancy clusters; including boat vacancy (V_4~b) and zig-zag vacancy (V_4~Z). The results show that the presence of unstable V3 vacancy may induce the mono-vacancy to migrate with low energy and vanish through forming stable V4 vacancy cluster. Also, the calculated energy barriers required to form the boat vacancy cluster (V_4~b), the zig-zag vacancy cluster (V_4~Z) and disk vacancy cluster ( V4) support that the disc and the boat vacancy clusters co-exist. However the zig-zag type might only exist by knocking-out mechanism for highly irradiated graphite.
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