Election paramagnetic resonance studies on Al centers in vitreous silica

Two new aluminum spectra have been observed with EPR in irradiated vitreous silica. Under electron irradiation at 40K, the low temperature (LT) Al spectrum is produced, and it changes into the high temperature (HT) spectrum upon annealing between 230 and 3300K. The HT Al spectrum is also produced directly upon irradiation at room temperature. Each spectrum is characterized by six well resolved 27Al (I = 5/2) hyperfine lines (A/sub isotropic/(LT) = 0.00330 cm-1 and A/sub isotropic/(HT) = 0.0410 cm-1). These spectra are the most intense in type I and II vitreous silicas which contain Al as a natural impurity. The difference in these spectra may reflect the presence of a neighboring charge compensator, such as an alkali, which becomes mobile at approximately equal to 2700K. It is possible that these spectra correspond to Al E1'-type centers with the unpaired electron strongly localized on a substitutional Al which is bonded to only three oxygen atoms