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In this research, formation mechanisms of Fe(Mg)O.Al2O3 solid solution synthesizing by co-precipitation method was investigated. To this aim, a mixed solution of corresponding amount of aluminum chloride hexahydrate (AlCl3.6H2O), iron (II) chloride tetrahydrate (FeCl2.4H2O) and magnesium chloride (MgCl2) with Mg2+: Fe2+: Al3+ molar ratio of 1:1:2 in distilled water at 60 °C was prepared. Afterward, a gelatinous precipitate was formed with excess of NaOH solution at pH 9.5–10.5. The produced precipitate was dried at 110 °C for 24 h and characterized by Fourier transform infrared (FTIR) spectroscopy, while its thermal behavior was studied by simultaneous thermal analysis (STA). In order to explain the thermal behavior, different samples were calcined at different temperatures in air atmosphere, and their phase compositions were characterized by X-ray diffraction (XRD) analysis. Moreover, morphologies of samples were evaluated by scanning electron microscope (SEM). Results revealed that in-situ spinel was formed via the reaction between MgO and γ-Al2O3 at a temperature of 800ºC. The produced spinel create appropriate conditions for formation of Fe(Mg)O.Al2O3, and a solid solution based on Hercynite ((Fe,Mg)O.Al2O4) was formed. In addition, the morphology of formed solid solution depended on the in-situ spinel, and it seems that solid solutions with column structure was formed via the evolution of the spinel, forming via the reaction between MgO and γ-Al2O3‌, and those with tabular or octahedral shapes was formed via the reaction between MgO and α-Al2O3.

Keywords: Hercynite, Formation mechanisms, Spinel, Co-Precipitation, Magnesium chloride.

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