THE MANY VARIETIES OF E' CENTERS - A REVIEW

Three varieties of E' centers with a spin state, S = 1/2, and with a G-tensor, G(x) similar to G(y) similar to 2.0003, G(Z) similar to 2.0018, were identified in the early reports on paramagnetic states in irradiated alpha-quartz. The atomic structure of two of these had an hydrogen ion (proton) in nearby sites and hence there are hyperfine interactions of the E' electrons with the nuclear moments of the hydrogens, I = 1/2. These three types have not yet been resolved in the spectra of silicas. In addition, two other variants on the oxygen vacancy configuration of the E' center have been observed in silica. In alpha-quartz, four varieties of E'' centers have been observed for which S = 1. One of the these has been resolved in the spectra of some silicas. The progressive reduction of SiO2 to SiO(2-x) for which 0 < x < 2 introduces E'-type centers whose mean G-value increases from similar to 2.0010 to similar to 2.0055 and with a width of similar to 0.05 mT. The 2.005 G-value is that observed for paramagnetic electrons in amorphous Si. The spectra of these states are resolved from the E' spectrum when x > 1.38. This E' type is observed in ion-implanted silica.