Electron paramagnetic resonance of Al E1/sup prime/ centers in vitreous silica

A new series of 27Al 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< or approx. =260 K), pre-existent, diamagnetic network defects of the form AlO/sub 3/2/ trap an electron to form paramagnetic AlO/sub 3/2//sup ts-/. Upon annealing or irradiation at 300 K, AlO/sub 3/2//sup ts-/ traps a compensator such as Na+, Li+, or H/sub n//sup ts+/. With the aid of high-temperature electrolysis, specific spectra are associated with specific compensators. In one spectrum a superhyperfine interaction with an isotope having I=3/2 and attributed to 23Na is observed. We observe that the role of charge compensators with respect to the charge (magnetic) state of these Al defects in SiO2 gives a consistent indication of the aluminum-oxygen coordination. We also demonstrate a correlation between the strength of the 27Al contact hyperfine interaction, the presence of charge compensators, and the thermal stability of the trapped electron. An analysis of the 27Al and 29Si hyperfine interactions arising from paramagnetic AlO/sub 3/2//sup ts-/ and SiO/sub 3/2/ (E1/sup prime/) centers indicate that these two centers are isoelectronic. Each of the four 27Al 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 E1/sup prime/ centers might be incorporated in the v-SiO/sub 4/2/ network are also presented. In particular, a model for an extrinsic Si E1/sup prime/ center is proposed which may explain various effects observed with EPR in different kinds of v-SiO260CO γ irradiated with fluences < or approx. =107 R