Redox kinetics of iron in alkali silicate glasses exposed to ionizing beams: Examples with the electron microprobe

We report herein on changes in the Fe3+/ΣFe ratios induced by the electromigration of alkali ions (Alk+s) in natural alkali silicate glasses by measuring the shift of the Fe Lα emission peak at the electron microprobe. The Fe3+ reduction to Fe2+ classically occurs by electron transfer (Fe3+ + e− → Fe2+). The inward diffusion (to the bulk) of Alk+s is correlated with the outward diffusion (to the surface) of electrons transferred from a Fe2+ site to a neighboring Fe3+ site. This reduction process is somewhat different when iron is found at low amounts in glasses. In the latter case, Fe3+ is an efficient electron trap and its reduction to Fe2+ occurs by direct capture of a free electron. The Fe2+ oxidation is induced by the formation and the outward diffusion of O2− interstitial ions produced at the sites of paired nonbridging oxygens after the departure of the charge compensating Alk+s. The accumulation of free oxygens beneath the surface makes Fe3+-rich oxide phases to precipitate as separate nanometer sized particles. Outgassing of atomic oxygens as bubbles is also observed.

► We investigate damage induced on natural alkali glasses bombarded by electrons.
► Iron redox changes are correlated to alkali diffusion and oxygen outgassing.
► At low (high) alkali amounts iron reduction (oxidation) is observed.
► Fe3+ to Fe2+ reduction occurs by electron transfer (Fe3+ + e− → Fe2+).
► Fe2+ to Fe3+ oxidation leads to the formation of iron oxides at the glass surface.