Control of the Crystal Phase Composition of FexOy Nanopowders Prepared by CO2 Laser Vaporization

Crystalline magnetic iron oxide nanopowders are prepared by CO2 laser vaporization (LAVA) of a hematite (a-Fe2O3) raw powder. Condensation at normal pressure leads to maghemite (g-Fe2O3) as the main phase in the nanopowders. With increasing oxygen partial pressure in the zone of condensation an increasing amount of the Fe2O3 polymorph e-Fe2O3 is found. The LAVA prepared Fe2O3 nanopowders are characterized by X-ray diffraction, transmission electron microscopy, chemical analysis, and with respect to their magnetic properties. A mechanism for the initial nucleation is proposed explaining the formation of e-Fe2O3 with increasing oxygen content of the condensation atmosphere. The model is based on the evidence of ozone in oxygen-rich condensation atmospheres. Density functional theory calculations indicate that ozone facilitates the formation of 6-fold oxygen coordinated Fe ions acting as building units for the emerging crystal structure during the solidification of the nanoparticles. This insight into early nucleation stages will be useful for the functional design and crystal engineering of either isostructural materials like alumina (Al2O3) or, more generally, vapor phase synthetic routes for ceramic materials.