Hydrothermal phase transformation of hematite to magnetite

Different phases of iron oxide were obtained by hydrothermal treatment of ferric solution at 200°C with the addition of either KOH, ethylenediamine (EDA), or KOH and EDA into the reaction system. As usually observed, the α-Fe₂O₃ hexagonal plates and hexagonal bipyramids were obtained for reaction with KOH and EDA, respectively. When both KOH and EDA were added into the reaction system, we observed an interesting phase transformation from α-Fe₂O₃ to Fe₃O₄ at low-temperature hydrothermal conditions. The phase transformation involves the formation of α-Fe₂O₃ hexagonal plates, the dissolution of the α-Fe₂O₃ hexagonal plates, the reduction of Fe³⁺ to Fe²⁺, and the nucleation and growth of new Fe₃O₄ polyhedral particles.