Glass–crystalline materials with high iron oxide concentration: Phase composition, redox ratio and magnetic properties

The synthesis of glass–crystalline materials in the system Na₂O/CaO/SiO₂/Fe₂O₃ with high concentrations of Fe₂O₃ (20, 25 and 30 mol%) by applying the melt-quenching technique is reported. The melts spontaneously crystallize during pouring and the formation of magnetite (Fe₃O₄), hematite (α-Fe₂O₃) and ɛ-Fe₂O₃, as identified by X-ray diffraction (XRD) is observed. The microstructure and the elemental composition of the prepared materials are further investigated by scanning electron microscopy (SEM) and two different morphological types of Fe-containing crystals – needle-like and dendrite-shaped are detected. Mössbauer spectroscopy showed the presence of Fe³⁺ and Fe²⁺, as well as the existence of iron ions both in tetrahedral and octahedral coordination and the precipitation of hematite, ɛ-Fe₂O₃ and magnetite. The magnetic measurements on the prepared samples reveal ferrimagnetic properties with well defined hysteresis curves, although due to relatively small volume fraction of the iron-rich crystalline phases, the net magnetic moment is quite low compared to the bulk values for magnetite.