Diluted Fe3+ in silicate glasses: Structural effects of Fe-redox state and matrix composition. An optical absorption and X-band/Q-band EPR study

• First data on iron site distortion and distribution and inter-site partitioning.
• Fe3+ in reduced glass exhibits original properties.
• In Na(Ca, Mg) glasses, Fe3+ has a preference for Ca relative to Mg neighbors.
• Fe3+ optical transitions are explained by the presence of both 4- and 5-fold sites.
• Presence of Fe-rich clusters confirms the heterogeneous structure of glasses.

Sodium-alkaline earth-silicate glasses, of nominal molar composition 16Na2O–10RO–74SiO2 (R = Ca, (Ca,Mg) and Mg) doped with 0.5 wt.% of Fe2O3, were studied by UV–Visible–NIR absorption spectroscopy and electron paramagnetic resonance (EPR) at X- and Q-band to understand the structural control of Fe3+ optical absorption properties as a function of iron redox and glass composition. By comparing with a set of [4]Fe3+, [5]Fe3+ and [6]Fe3+ crystalline references, optical absorption spectra indicate the presence of 5-fold Fe3+ in addition to a majority of tetrahedral Fe3+. The combination of Q- and X-band EPR data shows Fe3+ partition among isolated, distributed sites and Fe-rich clusters, providing unique insight into the distortion of isolated Fe3+ sites. It demonstrates also the peculiar character of the residual Fe3+ sites that exist in reduced glasses. Changing Ca to Mg increases the amount of tetrahedral Fe3+ sites and decreases their distortion. The presence of Mg also reduces the amount of isolated rhombic Fe3+ sites and promotes the formation of clusters. These clusters confirm the non-homogeneous structure of silicate glasses, as well as the preference of Fe3+ for a more calcic than magnesian environment in sodic (Ca,Mg) glasses.