The geometries, formation energies and diffusion barriers of C point defects in α-quartz were calculated by using a charge self-consistent density-functional based non-orthogonal tight-binding method. It was found that bonded interstitial C configurations had significantly lower formation energies (of the order of 5eV) than did substitutionals. The activation energy of atomic C diffusion, via trapping and de-trapping in interstitial positions, was about 2.7eV. The extraction of a CO molecule required an activation energy of less than 3.1eV, but the CO molecule could diffuse with an activation energy of less than 0.4eV. Re-trapping in O vacancies was hindered by a barrier of about 2eV.

Theoretical Investigation of Carbon Defects and Diffusion in α-Quartz. C.Köhler, Z.Hajnal, P.Deák, T.Frauenheim, S.Suhai: Physical Review B, 2001, 64[8], 085333