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

• Published in: Journal of non-crystalline solids Vol. 262; no. 1; pp. 35 - 65
• Main Authors: Shkrob, Ilya A., Tadjikov, Boris M., Trifunac, Alexander D.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-02-2000; Elsevier
• Subjects: AXIAL SYMMETRY; Color centers and other defects; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Electron paramagnetic resonance and relaxation; ELECTRON SPIN RESONANCE; ENDOR; Exact sciences and technology; GLASS; IRRADIATION; Magnetic resonances and relaxations in condensed matter, mössbauer effect; MATERIALS SCIENCE; Physics; POINT DEFECTS; RADIATION EFFECTS; Radiation effects on specific materials; RADICALS; RELAXATION; SILICATES; Structure of solids and liquids; crystallography; A140; N220; 76.70.Dx; R100; 61.82.Ms; 76.30.Mi; 71.23.Cq; Inorganic compounds; Point defects; ESR; Physical radiation effects; Metastable states; Glass; Electron beam; Spin echo; Experimental study; ENDOR; Dangling bonds; Trapping; Gamma radiation; Simulation; Holes; Ab initio calculations; Semiempirical method
• ISSN: 0022-3093; 1873-4812
• DOI: 10.1016/S0022-3093(99)00669-9

Irradiation of alkali silicate glasses results in the formation of metastable spin centers, such as oxygen hole centers (OHC
1 and OHC
2), 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 OHC
1 centers the ≡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 OHC
2 is the result of rapid tunneling of the electron between two degenerate sites with rate
(0.2−50)×10
11
s
−1
and activation energy ∼10 meV. This center is a hole trapped on a tetrahedral >SiO
2
2− unit or a planar –SiO
2
− 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.