Wigner defects bridge the graphite gap

We present findings on the structure, energies and behaviour of defects in irradiated graphitic carbon materials. Defect production due to high-energy nuclear radiations experienced in graphite moderators is generally associated with undesirable changes in internal energy, microstructure and physical properties—the so-called Wigner effect. On the flip side, the controlled introduction and ability to handle such defects in the electron beam is considered a desirable way to engineer the properties of carbon nanostructures. In both cases, the atomic-level details of structure and interaction are only just beginning to be understood. Here, using a model system of crystalline graphite, we show from first-principles calculations, new details in the behaviour of vacancy and interstitial defects. We identify a prominent barrier-state to energy release, reveal a surprising ability of vacancy defects to bridge the widely spaced atomic layers, and discuss physical property and microstructure changes during irradiation, including interactions with dislocations.