Structure and redox ratio of soda-lime-silica glasses with high iron oxide concentrations

The high temperature synthesis of glasses in the system (100-x)(0.16Na₂O/0.10CaO/0.74SiO₂)/xFe₂O₃, x = 5 ÷ 20 mol% is reported. For Fe₂O₃ concentrations ≤15 mol%, glasses are formed while the sample with 20 mol% crystallizes during cooling the melt. X-ray diffraction shows the crystallization of magnetite. The microstructure of the glass-crystalline sample is investigated by optical microscopy and scanning electron microscopy and two types of iron-rich crystals corresponding to magnetite and hematite are detected. The refractive indices as determined by the Becke line method are in the 1.567 - 1.639 range and. increase with increasing Fe₂O₃ concentration. The structure is characterized using Infra-red spectroscopy. The presence of symmetric stretching, asymmetric stretching and bending vibrations of Si-O-Si is detected and attributed to the occurrence of SiO₄ tetrahedral units with varying numbers of nonbridging oxygens. Also, the increasing Fe₂O₃ concentration results in occurrence of Fe-O-Si bonds indicating the glass network depolymerization due to Fe₂O₃ addition. In all samples, the presence of Fe³⁺ and Fe²⁺ and the existence of iron ions in tetrahedral and octahedral coordination, as well as a very small amount of Fe⁰ and the precipitation of hematite and magnetite in the glass-crystalline sample is revealed by Mössbauer spectroscopy.