International audienceThe nature of the physical mechanisms related to the gamma-Fe2O3 to alpha-Fe2O3 phase transition under laser irradiation and heat treatment has been investigated using in situ micro-Raman spectroscopy and X-ray powder diffraction (XRPD) analysis. Measurements were carried out on as-prepared gamma-Fe2O3 nanoparticles of about 4 nm in size as a function of laser power and on annealed gamma-Fe2O3 particles. Annealing temperature affects the relative fractions of the gamma-Fe2O3 and alpha-Fe2O3 phases, and at 450 degrees C, the phase transition into alpha-Fe2O3 becomes complete with apparent crystallite size < D > of about 30 nm. The hematite nanoparticles increase then up to more than 180 nm at 1400 degrees C. The excellent agreement between evolution of the wavenumbers and bandwidths confirms that the heat treatment and laser irradiation produces the same effects on nanoparticles. Correlations between structure modifications occurring at the nanometric scale during grain coalescence and the evolution of Raman vibrational spectra were quantitatively examined, and a physical mechanism for the gamma -> alpha-Fe2O3 phase transition was proposed

International audienceThe nature of the physical mechanisms responsible for the structural modification of the γ-Fe2O3 nanoparticles under laser irradiation has been investigated by Raman spectroscopy. In situ micro-Raman measurements were carried out on as-prepared γ-Fe2O3 nanoparticles about 4 nm in size as a function of laser power and on annealed γ-Fe2O3 particles. A baseline profile analysis clearly evidenced that the phase transition from maghemite into hematite is caused by local heating due to laser irradiation with an increase of grain size of nanoparticles. This increasing was clearly determined by X-ray diffraction from 4 nm in nanoparticles up to more than 177 nm beyond 900 °C in a polycrystalline state