Carbon Interstitial in the Diamond Lattice

Linear-combination-of-atomic-orbitals-molecular-orbitals cluster calculations using the extended H\"uckel theory are carried out for the interstitial carbon in the diamond lattice. The results suggest that the interstitial configuration is not the tetrahedral or hexagonal site, as has been previously assumed, but is instead an "interstitialcy" configuration, i.e., either a split- 〈100〉 interstitial (which our results favor) or a bond-centered interstitial. The predicted minimum-energy configuration changes with charge state, suggesting that the interstitial in the diamond lattice is a possible example of the Bourgoin mechanism of athermal migration of a defect in the presence of ionizing radiation.