Structural and chemical evolution of FeCoO based ceramics under reduction/oxidation—an in situ neutron diffraction study

Air-sintered ceramic samples in composition CoO·nFe2O3 (n=1 and 2) were prepared in solid-state reactions, resulting in a single spinel phase with composition CoFe2O4, and a two-phase mixture of identical spinel with α-Fe2O3 for n=1 and 2, respectively. Their structural and chemical evolution over pO2 range of 10−0.9 to 10−19 atm was investigated using in situ neutron diffraction at isothermal condition (∼900 °C). Neutron diffraction data were analyzed through Rietveld refinements. The following sequences of structural transformation from α-Fe2O3 hematite→(Fe,Co)-spinel→(Fe,Co)1−xO wustite→(Fe,Co)O rocksalt→γ-(Fe,Co)→α-(Fe,Co)→(γ-(Fe,Co)) were observed on the reduction of FeCoO based ceramics. With the development of reduction at pO2 down to 10−15 atm, mixed valence (Fe2+ and Fe3+) spinel was first formed in Fe-excess (Fe, Co) spinel phases. The intermediate phases were usually Co-rich compared with their parent mixed oxide phases. Particularly, the initial metallic precipitate is Co-rich γ, independent of initial stoichiometry. Reduction kinetics at pO2 of ∼10−19 atm is extremely fast, but crystalline form and structural integrity are maintained. As crystal structures of the various involved phases are very similar, few structural blocks were disturbed as oxygen was released from the samples. In addition, re-oxidation behavior of reduced products was also studied, and phase composition and microstructure of post-neutron experiments were characterized by X-ray diffraction and scanning electron microscopy.