Amorphous iron-chromium oxide nanoparticles prepared by sonochemistry

Amorphous Fe₂O₃ and Fe_1.9Cr_0.1O₃ materials have been prepared by sonochemical method. X-ray diffraction patterns, transmission electron microscopy, Raman and infrared spectra, differential scanning calorimetry, Mössbauer and magnetic measurements revealed many interesting behaviors of the samples. Reaction to form the materials only occurred at the preparation temperatures of 70 °C or above. Upon heating, the sample prepared at 70°C presented a strong ferromagnetic behavior due to the presence of the magnetite phase coexisting with the hematite phase whereas the samples prepared at higher temperatures presented only the existence of the hematite phase. Thermal analyses of the sample prepared at 80°C revealed three exothermic peaks which were corresponding to the phase changes of dehydroxylation, crystallization of the maghemite phase and maghemite–hematite transition, respectively. The activation energies of the phase changes deduced from the thermal analyses showed that the presence of Cr enhanced the activation energy which can slow down the ageing effect of the amorphous state when being used in practice.