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LaFeO3 perovskite was synthesized mechanochemically through ball milling of La2O3 and Fe2O3 in stoichiometric ratio. X-ray powder diffraction (XRPD), simultaneous differential scanning calorimetry and thermogravimetry analysis (DSC–TGA), Mossbauer spectroscopy, scanning electron microscopy (SEM), and optical diffuse reflectance spectroscopy were combined for a detailed study of the growth mechanism of LaFeO3 perovskite during the ball milling process. The XRPD results showed that La2O3 is unstable when exposed to air. Both La2O3 and La(OH)3 phases coexist under the ball milling process, indicating that La2O3 or La(OH)3 can be used to produce LaFeO3. The formation of LaFeO3 perovskite was evident after only 2 h of milling and the amount of LaFeO3 gradually increased with the increase of ball milling time. After 12 h of ball milling, single phase LaFeO3 was formed. The Mossbauer spectroscopy studies show that the spectrum of the formed LaFeO3 phase consists of three sextets and one doublet, indicating the wide distribution of LaFeO3 particle sizes and some of the smaller particles having superparamagnetic properties. This is in good agreement with the SEM images, which show that the formed LaFeO3 phase consists of nanometer-sized particles and micrometer-sized agglomerates. The formation of LaFeO3 phase was mainly caused by the La3+ substitution of Fe3+ in Fe2O3 lattice. Optical diffuse reflectance spectroscopy studies show that the formed LaFeO3 phase has semiconductor properties, with the band gap energy ~2.67 eV.

Indium oxide-doped hematite xIn2O3·(1 − x)α-Fe2O3 (x = 0.1–0.7) nanostructure system was synthesized using mechanochemical activation by ball milling and characterized by XRD, simultaneous DSC–TGA, and UV/Vis/NIR. The microstructure and thermal behavior of as obtained system were dependent on the starting In2O3 molar concentration x and ball milling time. XRD patterns yielded the dependence of lattice parameters and grain size as a function of ball milling time. After 12 h of ball milling, the completion of In3+ substitution of Fe3+ in hematite lattice occurs for x = 0.1, indicating that the solid solubility of In2O3 in hematite lattice is extended. For x = 0.3, 0.5, and 0.7, the substitutions between In3+ and Fe3+ into hematite and In2O3 lattice occur simultaneously. The lattice parameters a and c of hematite and lattice parameter a of indium oxide vary as a function of ball milling time. The changes of these parameters are due to ion substitutions between In3+ and Fe3+ and the decrease in the grain sizes. Ball milling has a strong effect on the thermal behavior and band gap energy of the as-obtained system. The hematite decomposition is enhanced due to the smaller hematite grain size. The crystallization of hematite and In2O3 was suppressed, with drops of enthalpy values due to the stronger solid–solid interactions after ball milling, which caused gradual In3+–Fe3+ substitution in hematite/In2O3 lattices. The band gap for hematite shifts to higher energy value, while that of indium oxide shifts to lower energy value after ball milling.