Magnetic resonance studies on radiation-induced point defects in mixed oxide glasses. II. Spin centers in alkali silicate glasses

Irradiation of alkali silicate glasses results in the formation of metastable spin centers, such as oxygen hole centers (OHC1 and OHC2), silicon peroxy radicals and a silicon dangling bond (E′ center). In this work, electron paramagnetic resonance (EPR) and electron nuclear double magnetic resonance (ENDOR) are used to study these spin-1/2 defects. It is shown that in a subset of the OHC1 centers the ≡SiO  radical is strongly coupled to a single alkali cation. Thermally activated swinging motion of this cation causes asymmetric T2 relaxation and changes electron spin echo envelop modulation (ESEEM) spectra. It is argued that trapping of the hole by non-bridging oxygen atoms does not result in the release of a compensating alkali cation. Rather, the O–Alk bond elongates and the whole structure relaxes. This view is supported by semi-empirical and ab initio calculations. The observed axial symmetry of the g-tensor for OHC2 is the result of rapid tunneling of the electron between two degenerate sites with rate (0.2−50)×1011 s−1 and activation energy ∼10 meV. This center is a hole trapped on a tetrahedral >SiO22− unit or a planar –SiO2− unit. It is demonstrated that silicon peroxy radicals are not formed by charge trapping. Their generation is temperature-independent and occurs via the decay of self-trapped excitons. It seems likely that the same process yields silicon dangling bond centers.