Quantum-mechanical modeling was used to study the effect of local and long-range disorder on the incorporation energy, optical, infra-red and XPS spectra of a peroxy linkage defect in amorphous silica. Methods of simulating amorphous structures were considered, and the extent to which cluster models were applicable to the study of a-SiO2 was investigated. For this purpose, the results of the periodic density-functional theory calculations of the possible peroxy linkages formed on incorporating an O atom in the amorphous super-cell were compared with the results obtained by using molecular and embedded cluster models. Simple molecular clusters terminated by H atoms were used, and larger clusters were treated by using the ONIOM technique, in which the distortion of the amorphous structure surrounding the defect was accounted for in a region of approximately 1.5nm. The results demonstrated the importance of the local and medium-range disorder around the defect, such as the peroxy center, which induces relaxation in an area of the amorphous structure extending beyond the first and second neighbors. The geometric structures and the distribution of incorporation energies of such a defect could not be adequately described in a simple molecular cluster model. On the other hand, optical transitions localized on the –O–O– bridge itself were not strongly affected by disorder. The results indicated that embedded cluster approaches, such as ONIOM, could provide a useful alternative to fully ab initio periodic calculations of geometrical structures and incorporation energies of point defects in silica.

Modeling Disorder in Amorphous Silica with Embedded Clusters - the Peroxy Bridge Defect Center. D.Ricci, G.Pacchioni, M.A.Szymanski, A.L.Shluger, A.M.Stoneham: Physical Review B, 2001, 64[22], 224104 (8pp)