Modeling disorder in amorphous silica with embedded clusters: The peroxy bridge defect center

We use quantum-mechanical modeling to study the effect of local and long-range disorder on the incorporation energy, optical, IR, and XPS spectra of a peroxy linkage defect in amorphous silica (a-SiO2). We discuss the methods to simulate amorphous structures and investigate to which extent cluster models are applicable to study a-SiO2. For this purpose we compare the results of the periodic DFT calculations of the possible peroxy linkages formed on incorporating an oxygen atom in the amorphous super-cell with the results obtained using molecular and embedded cluster models. We use simple molecular clusters terminated by H atoms and larger clusters treated using the ONIOM technique, in which the distortion of the amorphous structure surrounding the defect is accounted for in a region of approximately 15 Angstrom. The results demonstrate 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 cannot be adequately described in a simple molecular cluster model. On the other hand, optical transitions localized on the -O-O- bridge itself are not strongly affected by disorder. Our results indicate that the embedded cluster approaches, such as ONIOM, can provide a useful alternative to fully ab initio periodic calculations of geometrical structures and incorporation energies of point defects in silica.