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

摘要

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 drop SiO . radical is strongly coupled to a single alkali cation, Thermally activated swinging motion of this cation causes asymmetric T-2 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) 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 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. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 109]