Electron trap centers in Ag-borate glasses

The center created by silver trapping an electron in borate glasses are considered. It is shown by ESR techniques that on trapping an electron, silver ions may form four types of centers: the single Ag⁰ or (Ag⁺ + e) centers, and, at higher Ag contents, the Ag⁰(Ag⁺) or (Ag⁺ + e) (Ag⁺) centers in which a second silver is involved. The formation and stability of these centers is critically dependent on the silver content, the concentration of alkali and the temperature of irradiation and measurement. The Ag⁰(Ag⁺) center is generally enhanced by increasing silver and decreasing alkali content and does not show separate signals for the two isotopes ¹⁰⁷Ag, ¹⁰⁹Ag because the two silvers in proximity occur in three different isotope combinations which are superimposed.Above 20% alkali the hyperfine splitting constant for the silver—electron trap center was found to decrease with increasing alkali content, probably in connection with the increasing appearances of non-bridging oxygens and the trapping of the electron in an oxygen vacancy in the vicinity of Ag⁺, i.e. a change from Ag⁰ to (Ag⁺ + e) center. A similar decrease was observed in high alkali (30% R₂O) glasses with changes of alkali from Li⁺ to K⁺.