57Fe Mössbauer effect in sol-gel silicate glasses

Abstract

Mössbauer spectroscopy was used to study the iron in silica glasses, either pure, or containing 5 mol% Li 2O, Na 2O or CaO. Glasses with 5 mol% FeO and 1 mol% FeO were prepared by the sol-gel method. After drying at 60°C, gels were heat treated between 500 and 1000°C, under oxidizing or reducing conditions. Mössbauer spectra were taken at 300 and 80 K. Estimated isomer shifts for the spectra of all samples heated between 500 and 850°C in oxidizing conditions suggest that Fe 3+ is mainly in tetrahedral sites if a glass modifier is present. In pure ion silicate glasses, finely dispersed colloidal hematite is observed and Fe 3+ appears in both tetrahedral and octahedral sites. Under reducing conditions, below 500°C, all iron was found in alkali or CaO containing glasses as tetrahedral Fe 3+. Fe 2+, in octahedral sites, was only observed after heating above 600°C; the Fe 2+/Fe 3+ ratio increased with temperature and total iron content of the glasses. In the pure iron silicate samples, under reducing conditions, as temperature increased, Fe 3+ seemed to move from tetrahedral to octahedral sites.