Solubility limit and phase transformation in Si x Fe3− x O4 solid solution powders

The relationship between the particle size of Si-doped Fe3O4 powder and its phase transformation under heat treatment in air was systematically investigated. The solid solution powder was synthesized via the solid-state reaction method. X-ray diffraction analyses indicated that the lattice constant of Fe3O4 decreased with increasing Si content and then stabilized, suggesting a solubility limit of approximately x = 0.1 (SixFe3−xO4). Raman spectroscopy of powders subjected to heat treatment in air demonstrated that Fe3O4 exhibits oxidation resistance at particle sizes on the order of tens of micrometers, while the phase-transformed γ-Fe2O3 retains thermal stability even at 873 K for sub-micrometer particles. Cross-sectional examination of powder annealed in air at 773 K revealed that Si was incorporated into the phase-transformed γ-Fe2O3. Therefore, despite the relatively low solubility limit of Si in Fe3O4, the doped phases, particularly γ-Fe2O3, exhibit remarkable thermal stability.