Monitoring by Mossbauer spectroscopy the thermal reduction of hematite into magnetite: the surface effect and charge disproportionality in iron oxide nanoparticles

Nanoparticles of magnetite Fe3O4 were synthesized by thermal reduction of hematite alpha-Fe2O3 powder in the presence of high boiling point solvent. The structural transformations and magnetic properties of the obtained nanoparticles were investigated by the Fe-57 Mossbauer spectroscopy, X-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at each step of the chemical and thermal treatment of the product. An increase of saturation magnetization with the reaction time correlates with an increase of concentration of magnetite in the samples. The electron hoping between Fe2+ and Fe3+ ions in the octahedral sites of the magnetite nanoparticles and Verwey phase transition were investigated. It was established that not all iron ions in the octahedral sites participated in electron hoping Fe2+ reversible arrow Fe3+ above the Verwey temperature T-V, and the charge distribution could be expressed as (Fe3+)(tet)[(Fe1.85Fe0.153+)-Fe-2.5](oct) O-4.