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A rich variety of phenomena are observed when graphite is exposed to high doses of radiation from energetic particles. Most notably, the crystals expand along their c-axes, and dimension changes of tens of percent or even more are easily achieved. There are significant changes to the thermal and electrical properties of the material as well. When irradiation occurs below about 400 K, energy accumulates in the material, and the amount can be large in proportion to the specific heat. Known as Wigner energy, this is released by annealing, and is accompanied by a partial reversal of the initial changes, including conservation of the crystal volume. Nevertheless, the original dimensions of the crystals are not restored. The origins of this behaviour are spread over the atomic, nanometre, and mesoscale. They lie in the generation of Frenkel pairs and, we argue, dislocations. Models based on density functional theory provide insight into the likely nature and evolution of the defect structure during and after radiation.

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