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

We report herein on changes in the Fe 3+ /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 Fe 3+ reduction to Fe 2+ classically occurs by electron transfer (Fe 3+ + e − → Fe 2+ ). The inward diffusion (to the bulk) of Alk + s is correlated with the outward diffusion (to the surface) of electrons transferred from a Fe 2+ site to a neighboring Fe 3+ site. This reduction process is somewhat different when iron is found at low amounts in glasses. In the latter case, Fe 3+ is an efficient electron trap and its reduction to Fe 2+ occurs by direct capture of a free electron . The Fe 2+ oxidation is induced by the formation and the outward diffusion of O 2− 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 Fe 3+ -rich oxide phases to precipitate as separate nanometer sized particles. Outgassing of atomic oxygens as bubbles is also observed.