Iron-manganese hydroxycarbonate precursors in the synthesis of high-dispersity spinel mixed oxides

Iron-manganese hydroxycarbonates, obtained by coprecipitation, have been investigated as precursors for the synthesis of high-dispersity mixed oxides. Characterization was performed by X-ray diffraction (XRD), differential thermal analysis (DTA), IR and Mossbauer spectroscopy. Most of the iron cations are present in the trivalent state. In samples with atomic ratio Mn/Fe greater than or equal to 2 a rhodochrosite-like carbonate phase is present and the divalent cation form is favoured. A higher iron content leads to the formation of quasi-amorphous hydroxycarbonates. The thermal decomposition of precursors takes place below 620 K and monophasic spinel oxides are obtained, the manganese-rich samples being tetragonal. A number of exo- and endo-thermic transitions are detected above 770 K by DTA, due to phase segregation and recrystallization processes. Two-phase oxide products are obtained at 870 K [spinel + sesquioxide (Mn2-xFexO3)] for Mn/Fe greater than or equal to 1; and monophasic manganese ferrite with a unit-cell parameter of 0.841 nm. The formation of the spinel structure is preceded by a preliminary stage with octahedral symmetry dominating the cation coordination. The IR spectra of the mixed oxides indicate the presence of short-range ordered cation vacancies in the oxide phases.