Mossbauer spectroscopy, XRPD, and SEM study of iron-containing Na2O-B2O3-SiO2 glasses

Glasses in the Na2O-B2O3-SiO2-Fe2O3 system are suitable for producing magnetic porous glasses-host matrices for multiferroic composite materials for spintronics applications. Successful synthesis of such materials is dependent on the knowledge of crystallization and immiscibility areas in this system. Here, we report new findings for such glasses with a constant SiO2 concentration of 70 mol %, containing 7-12 mol % Fe2O3, whose compositions lie in the low (2 mol % Na2O) and higher (8-12 mol % Na2O) alkali regions, heat-treated at 550 degrees C. Glasses were studied using analytical chemistry methods and investigated by means of Mossbauer spectroscopy, XRPD and SEM. We outlined the immiscibility area boundary in the chosen silica cross-section for 550 degrees C, indicating the region of interconnected morphology and the crystallization fields of magnetite and FeSiO3, and correlated them with data on the valence and coordination state of iron in glasses. We find that both Fe3+ and Fe2+ ions in the low-alkali region are octahedrally coordinated, in higher alkali area Fe3+ and Fe2+ are tetrahedrally and octahedrally coordinated, respectively. A significant amount of Fe2+ in the low-alkali region can be the cause of the FeSiO3 formation. The usual crystalline phase in non-X-Ray amorphous glasses is magnetite, precipitating at the annealing stage.

Automated summary

Glasses in the Na2O-B2O3-SiO2-Fe2O3 system with compositions containing 7-12 mol % Fe2O3 and a constant SiO2 concentration of 70 mol % were studied at 550 degrees C. The immiscibility areas and crystallization fields of magnetite and FeSiO3 were outlined, with Fe3+ and Fe2+ ions found to be octahedrally coordinated in the low-alkali region and tetrahedrally and octahedrally coordinated in the higher alkali area. The formation of FeSiO3 in the low-alkali region was attributed to a significant amount of Fe2+.