We determine the structure and energetics of the point defects in graphite from first principles calculations. The Jahn−Teller effect takes place at low temperature, breaking the symmetry and lowering the vacancy formation energy. This results in a weak in− plane reconstruction bond between two of the unsaturated atoms surrounding the vacancy and the displacement of the remaining unsaturated atom out of the plane. This last feature of the distortion increases the interaction of the vacancy with other defects, affecting its migration energy. We comment on the STM images of the symmetric and asymmetric vacancy and also the significance of these findings in understanding defect behaviour in irradiated graphite and related graphitic materials, in particular single walled nanotubes.

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Ab initio density-functional calculations using Gaussian orbitals are carried out on large Si and Ge supercells containing oxygen defects. The formation energies, local vibrational modes, and diffusion or reorientation energies of O i , O 2i , VO, VOH, and VO 2 are investigated. The piezospectroscopic tensors for O i , VO, and VO 2 are also evaluated. The vibrational modes of O i in Si are consistent with the view that the defect has effective D 3d symmetry at low hydrostatic pressures but adopts a buckled structure for large pressures. The anomalous temperature dependence of the modes of O 2i is attributed to an increased buckling of Si-O-Si when the lattice contracts. The diffusion energy of the dimer is around 0.8 eV lower than that of O i in Si and 0.6 eV in Ge. The dimer is stable against VO or VO 2 formation and the latter defect has modes close to the reported 894-cm Ϫ1 band. The reorientation energies for O and H in VO and VOH defects are found to be a few tenths of an eV and are greater when the defect has trapped an electron.

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