ELECTRON-PARAMAGNETIC RESONANCE OF AL E1' CENTERS IN VITREOUS SILICA

A new series of Al27 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 260 K), pre-existent, diamagnetic network defects of the form AlO32 trap an electron to form paramagnetic AlO32-. Upon annealing or irradiation at 300 K, AlO32- traps a compensator such as Na+, Li+, or Hn+. 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=32 and attributed to Na23 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 Al27 contact hyperfine interaction, the presence of charge compensators, and the thermal stability of the trapped electron. An analysis of the Al27 and Si29 hyperfine interactions arising from paramagnetic AlO32- and SiO32 (E1′) centers indicate that these two centers are isoelectronic. Each of the four Al27 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′ centers might be incorporated in the v-SiO42 network are also presented. In particular, a model for an extrinsic Si E1′ center is proposed which may explain various effects observed with EPR in different kinds of v-SiO2 Co60 γ irradiated with fluences ≤107 R. © 1979 The American Physical Society.