First-principles total energy calculations were made of the ground-state properties and migration paths of interstitial C. The ground state involved 3-fold coordinated C and Si atoms, and its geometry suggested mainly  p  and  sp  bonding for C, rather than the  sp2  bonding that might have been expected. A consideration of the possible migration paths revealed that only 3 of them corresponded to small jumps which involved single bond-breaking. Of these, it was predicted that only 1 had a barrier of considerably lower energy (about 0.5eV) and involved an intermediate saddle-point configuration with C2 symmetry.

R.B.Capaz, A.Dal Pino, J.D.Joannopoulos: Physical Review B, 1994, 50[11], 7439-42