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.