Selective behaviour of dilute Fe3 + ions in silicate glasses: an Fe K-edge EXAFS and XANES study

Iron K-edge EXAFS and XANES have been used to study the local structural environment of dilute quantities (0.2 mol% Fe2O3) of Fe3 + in seven representative SiO2–R2O–R’O glasses where R = (Li, Na, K), R’ = (Mg, Ca, Sr, Ba). Sample preparation was carried out by adding excess CeO2 as an oxidising agent and it was confirmed by optical absorption spectroscopy that iron was fully oxidised to Fe3 +. This methodology has made possible, for the first time, an unambiguous EXAFS study of the coordination and local structural environment of dilute levels of Fe3 + in alkali–alkaline earth–silica glasses without the complications presented by the presence of Fe in multiple oxidation states. Coordination of Fe3 + in these glasses is strongly affected by the nature of the alkali and alkaline earth cations, with Fe3 +–O CN increasing from 4.0 ± 0.4 to 6.2 ± 0.9 with decreasing alkali/alkaline earth ionic radius ratio (IRR). Respective FeO bond lengths of 1.87 ± 0.01 Å to 1.92 ± 0.01 Å were also obtained. Dilute Fe3 + in the studied glasses displays selective behaviour, as demonstrated by the opposing effects of alkali and alkaline earth ions on measured Fe3 + CN and Fe3 +O bond length.