It was recalled that experimental results had led to the suggestion that the Eδ’

center was a 4-vacancy cluster rather than a single vacancy. Density functional

calculations were carried out here that explored the energetics and electronic

structure of single vacancies and clusters of four vacancies in realistic models of

amorphous silica. A total of 76 oxygen monovacancies and 38 four-vacancy

clusters were examined, and their energy levels and hyperfine parameters were

calculated. The results for single vacancies compared well with previous theory.

An important result for the four-vacancy clusters was that relaxations localized the

unpaired electron preferentially onto one Si atom; resulting in a strongly

anisotropic electron-paramagnetic-resonance signal. Electrons at single vacancies

had a less marked anisotropy, which was more compatible with the observed

isotropic signal.

Vacancy-Related Defects and the Eδ’ Center in Amorphous Silicon Dioxide:

Density Functional Calculations. B.R.Tuttle, S.T.Pantelides: Physical Review B,

2009, 79[11], 115206