Topological and bonding defects in vitreous silica surfaces

A model structure for an annealed silica surface was obtained through the molecular dynamics simulation technique employing three-body interaction potentials. Nonbridging oxygen and edge-sharing tetrahedra were found to form on the oxygen-terminated surface with three-coordinated silicon, three-coordinated oxygen, and three-membered rings just below the outermost atoms. Four-membered rings were also created in relatively large concentrations during the surface relaxation. When considering the effect of removing periodic boundary conditions along the z direction, the concentration of larger rings, relative to the bulk, remained unchanged throughout the ∼11 Å simulated surface region. A strong physical association between three-membered rings and three-coordinated oxygen was found which may account for the D2 defect peak observed in Raman scattering.