Formation of γ-Fe2O3 nanoparticles and vacancy ordering: An in situ X-ray powder diffraction study

The formation of maghemite, γ-Fe2O3 nanoparticles has been studied by in situ X-ray powder diffraction. The maghemite was formed by thermal decomposition of an amorphous precursor compound made by reacting lauric acid, CH3(CH2)10COOH with Fe(NO3)3·9H2O. It has been shown that cubic γ-Fe2O3 was formed directly from the amorphous precursor and that vacancy ordering starts about 45 min later at 305 °C resulting in a tripled unit cell along the c-axis. The kinetics of grain growth was found to obey a power law with growth exponents n equal to 0.136(6) and 0.103(5) at 305 and 340 °C, respectively. Particles with average sizes of 12 and 13 nm were obtained in 86 and 76 min at 305 and 340 °C, respectively. The structure of cubic and vacancy ordered phases of γ-Fe2O3 was studied at 305 °C by Rietveld refinements.