An embedded cluster method was used to predict and characterize the possible structural types of neutral and positively charged O vacancies in amorphous silica. Defects were treated at 70 different O sites of a continuous random network amorphous structure, generated by using classical molecular dynamics. The neutral vacancies were characterized by a wide distribution of formation energies and structural parameters. The modelling predicted the 2 main structural types of positively charged vacancy (E’ centers). That is, dimer and dangling bond centers. The local structure of both types of center depended upon the medium-range structure of the surrounding amorphous network. Most of the dangling bond centers were unpuckered. The so-called structural fingerprints, derived from similar calculations of O vacancy-type centers in quartz and from experiment, were used to find 2 other structural types of dangling bond center. These were the puckered configuration and the back-projected configuration of E’ centers. In each case, a distribution of both structural and electron paramagnetic resonance parameters was found. The average values of the electron paramagnetic resonance parameters for all dangling bond configurations were very similar. The structural criteria which favored the formation of different types of center in the original amorphous structure were formulated in terms of the average Si-O distance of an O ion with its 2 neighboring Si ions.

Modeling of the Structure and Properties of Oxygen Vacancies in Amorphous Silica. S.Mukhopadhyay, P.V.Sushko, A.M.Stoneham, A.L.Shluger: Physical Review B, 2004, 70[19], 195203 (2004)