Investigation of LaFeO_3 perovskite growth mechanism through mechanical ball milling of lanthanum and iron oxides

摘要

LaFeO_3 perovskite was synthesized mechanochemically through ball milling of La_2O_3 and Fe_2O_3 in stoichiometric ratio. X-ray powder diffraction (XRPD), simultaneous differential scanning calorimetry and thermogravimetry analysis (DSC-TGA), M?ssbauer spectroscopy, scanning electron microscopy (SEM), and optical diffuse reflectance spectroscopy were combined for a detailed study of the growth mechanism of LaFeO_3 perovskite during the ball milling process. The XRPD results showed that La_2O_3 is unstable when exposed to air. Both La_2O_3 and La(OH)_3 phases coexist under the ball milling process, indicating that La_2O_3 or La(OH)_3 can be used to produce LaFeO_3. The formation of LaFeO_3 perovskite was evident after only 2 h of milling and the amount of LaFeO_3 gradually increased with the increase of ball milling time. After 12 h of ball milling, single phase LaFeO_3 was formed. The M?ssbauer spectroscopy studies show that the spectrum of the formed LaFeO_3 phase consists of three sextets and one doublet, indicating the wide distribution of LaFeO_3 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 LaFeO_3 phase consists of nanometersizedparticles and micrometer-sized agglomerates. The formation of LaFeO_3 phase was mainly caused by the La~(3+) substitution of Fe~(3+) in Fe_2O_3 lattice. Optical diffuse reflectance spectroscopy studies show that the formed LaFeO_3 phase has semiconductor properties, with the band gap energy ～2.67 eV.