Structure and Hyperfine Parameters ofE1′Centers inα-Quartz and in VitreousSiO2

We report a first-principle study of the $E_1^{\prime }$ defect in $\alpha$-quartz and of the analogous $E_{\gamma }^{\prime }$ defect in amorphous ${\mathrm{SiO}}_2$. Our calculation supports the attribution of both these defects to a positively charged oxygen vacancy. The ground-state configuration of these defects is characterized by a large local relaxation of the atomic network, which leads to a localization of the unpaired electron on a Si dangling bond. Using the calculated electronic spin densities, we fully characterize the hyperfine interactions with nearby ${}^{29}\mathrm{Si}$. Our results explain well both the strong and the weak features that are observed in the experimental spectra.