Manganese ferrite synthesized from Mn(II) acetate + hematite freeze-dried powders

Freeze-drying was used to obtain a homogeneous fine powder containing (CH3COO)2Mn·2H2O and α-Fe2O3 (molar ratio Fe:Mn = 3:1), which was thermally treated for 2 h in N2 atmosphere, at 400 °C. This composite system was further annealed for 4 h in the same atmosphere, either at 700 °C or at 1000 °C. The 700 °C annealed sample was thermally treated in air at 1200 °C and then rapidly quenched. The phase transformations that occurred in the freeze-dried and heat-treated powders were investigated by thermal analyses, X-ray diffractometry, scanning electron microscopy and Mössbauer spectroscopy. Results showed that MnO–α-Fe2O3–Fe3O4 compounds were obtained after the first heat-treatment. Samples subsequently annealed in N2 atmosphere revealed the formation of Fe-rich manganese ferrites and manganowüstite phases. However, a single spinel phase was obtained for the sample that had been annealed and quenched in air.