Stable Vacancy Clusters in Neutron-Irradiated Graphite: Evidence for Aggregations with a Magic Number

Abstract

By combining positron-annihilation experiments and first-principles calculations, abundant planar ${V}_{6}$ rings are identified in heavily neutron-irradiated graphite. The calculations show that the ${V}_{6}$ ring is particularly stable. Annealing experiments exhibit that the ${V}_{6}$ ring survives up to $1500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. These results suggest vacancies aggregating with the magic number in graphite.