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 metastab le spin centers, such as oxygen hole centers (OHC1 and OHC2), silicon perox y 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 su bset of the OHC1 centers the drop SiO . radical is strongly coupled to a si ngle alkali cation, Thermally activated swinging motion of this cation caus es asymmetric T-2 relaxation and changes electron spin echo envelop modulat ion (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 catio n. Rather, the O-Alk bond elongates and the whole structure relaxes. This v iew 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 o f the electron between two degenerate sites with rate (0.2 - 50) x 10(11) s (-1) and activation energy similar to 10 meV. This center is a hole trapped on a tetrahedral >SiO22-:- unit or a planar -SiO2- unit. It is demonstrate d that silicon peroxy radicals are not formed by charge trapping. Their gen eration is temperature-independent and occurs via the decay of self-trapped excitons. It seems likely that the same process yields silicon dangling bo nd centers. (C) 2000 Elsevier Science B.V. All rights reserved.