Mössbauer studies of phosphate glasses for the immobilisation of toxic and nuclear wastes

Various iron-containing phosphate glasses were investigated by Mössbauer spectroscopy. Iron was found to occur predominantly as Fe3+ in all glasses, and largely occupied sites with distorted octahedral coordination for both redox states. Using a base glass of nominal composition 60 P2O5–40 Fe2O3 (mol%), stepwise molar replacement of Fe2O3 by (0.67 Na2O × 0.33 Al2O3) increased the redox ratio, Fe2+/ΣFe, from 0.13 at 40% Fe2O3 to 0.25 at 10% Fe2O3. The centre shift increased and quadrupole splitting decreased by up to ∼20% over this range, interpreted as a decrease in the average distortion of Fe sites from cubic symmetry, and an increase in average iron coordination. Literature revealed that recoil-free fraction ratio f (Fe3+) / f (Fe2+) ≈ 1.3 in iron phosphate glasses, and this was considered when assessing redox. Mössbauer parameters of these and other glasses demonstrated a combination of structural stability and compositional flexibility which makes them so suitable for waste immobilisation.