Radiation induced point defects in graphite and their phonon spectra

Energetics and vibrational modes of point defects that can be generated in the radiation damage process were studied in graphite using density functional theory. Di-Frenkel pairs formed from di-vacancies and two interstitials (at spiro or bridge) in cis and trans configurations were investigated in the graphite system. Di-Frenkel pairs with bridge interstitials in the trans and cis configurations were found to transform into crystallographic defects (CD). The formation of CD for cis configuration resulted in a substantial gain of energy. This stability is attributed to the formation of 4-55-77-8-membered rings after structural optimization, making them slightly more stable than monovacancies. Several CDs, including SW, are expected to form due to the diffusion of bridge interstitials into mono- and di-vacancies. The vibrational spectra due to the defects were calculated. Several defects generated additional high-frequency modes. Moreover, modes in which all atoms move in pristine graphite were split into modes with zero displacements to atoms in the layer of the defect or non-zero displacements to atoms in the adjacent layer.