Compositional dependence of infrared absorption of iron-doped silicate glasses

Peak positions of infrared absorption of Fe2+ were investigated for alkali alkaline-earth silicate and (alkali) alkaline-earth (alumino-)silicate glasses. The obtained data for absorption-peak wavenumber νp were analyzed as a function of calculated optical basicity Λc. For alkali alkaline-earth silicate glasses, increase of Λc by substitution of SiO2 for R′O increased νp, whereas increasing Λc by substituting Mg2+ for a larger alkaline-earth ion yielded the opposite trend. We consider that the composition changes vary effective negative charge on oxygen atoms and symmetry of their coordination around the Fe2+ ion. For (alkali) alkaline-earth (alumino-)silicate glasses, νp depended on the Na2O/Al2O3 ratio. ESR signals at g = 2.0 suggested clustering of Fe3+ and possibly Fe2+ ions in alkali-containing glasses and no clustering in alkali-free glasses. We consider that the negative charge of AlO4- is compensated preferentially by Fe3+ and Fe2+ ions when alkali cations are not present in glasses. Steep decrease of νp with increasing Λc by substituting Mg2+ for Ca2+ was observed in the regions with a very low or no MgO content for all series of glasses containing alkali ions. The effect was probably due to direct interaction between the Fe2+ ion and the Mg2+ ion.