Electron paramagnetic resonance of AlE1′centers in vitreous silica

A new series of ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ hyperfine spectra have been discovered by EPR studies in irradiated type-I and -II vitreous silica. Our analysis of these spectra indicate that under ionizing irradiation at low temperatures ($T\lesssim 260$ K), pre-existent, diamagnetic network defects of the form $\mathrm{Al}{\mathrm{O}}_{\frac{3}{2}}$ trap an electron to form paramagnetic $\mathrm{Al}{{\mathrm{O}}_{\frac{3}{2}}}^{-}$. Upon annealing or irradiation at 300 K, $\mathrm{Al}{{\mathrm{O}}_{\frac{3}{2}}}^{-}$ traps a compensator such as ${\mathrm{Na}}^{+}$, ${\mathrm{Li}}^{+}$, or ${\mathrm{H}}_n^{+}$. With the aid of high-temperature electrolysis, specific spectra are associated with specific compensators. In one spectrum a superhy-perfine interaction with an isotope having $I=\frac{3}{2}$ and attributed to ${}_{}{}^{23}{}_{}{}^{}\mathrm{Na}$ is observed. We observe that the role of charge compensators with respect to the charge (magnetic) state of these Al defects in Si${\mathrm{O}}_2$ gives a consistent indication of the aluminum-oxygen coordination. We also demonstrate a correlation between the strength of the ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ contact hyperfine interaction, the presence of charge compensators, and the thermal stability of the trapped electron. An analysis of the ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ and ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ hyperfine interactions arising from paramagnetic $\mathrm{Al}{{\mathrm{O}}_{\frac{3}{2}}}^{-}$ and $\mathrm{Si}{\mathrm{O}}_{\frac{3}{2}}$ (${E_1}^{\prime }$) centers indicate that these two centers are isoelectronic. Each of the four ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ hyperfine spectra are analyzed in terms of coaxial Zeeman and hyperfine interactions which include broadening effects due to variations in bond angles and lengths. Ideas about how Al and Si ${E_1}^{\prime }$ centers might be incorporated in the $v-\mathrm{Si}{\mathrm{O}}_{\frac{4}{2}}$ network are also presented. In particular, a model for an extrinsic Si ${E_1}^{\prime }$ center is proposed which may explain various effects observed with EPR in different kinds of $v$-Si${\mathrm{O}}_2$ ${}_{}{}^{60}{}_{}{}^{}\mathrm{Co}$ $\gamma$ irradiated with fluences ≤${10}^7$ R.