The geometries, formation energies, and diffusion barriers of carbon point defects in silica (α-quartz) have been calculated using a charge-self-consistent density-functional based nonorthogonal tight-binding method. It is found that bonded interstitial carbon configurations have significantly lower formation energies (on the order of 5 eV) than substitutionals. The activation energy of atomic C diffusion via trapping and detrapping in interstitial positions is about 2.7 eV. Extraction of a CO molecule requires an activation energy $<3.1\mathrm{}\hspace{0.5em}\mathrm{eV}$ but the CO molecule can diffuse with an activation energy $<0.4\mathrm{}\hspace{0.5em}\mathrm{eV}.$ Retrapping in oxygen vacancies is hindered—unlike for ${\mathrm{O}}_2—\mathrm{by}$ a barrier of about 2 eV.

• Received 11 April 2001

DOI:https://doi.org/10.1103/PhysRevB.64.085333