and X-ray Diffraction and Small-Angle X-ray Scattering Investigations of the Sol-Gel Synthesis of FeN and FeC.

Iron nitride (FeN) and iron carbide (FeC) nanoparticles can be prepared via sol-gel synthesis. While sol-gel methods are simple, it can be difficult to control the crystalline composition, , to achieve a Rietveld-pure product. In a previous synchrotron study of the sol-gel synthesis of FeN/FeC, we showed that the reaction proceeds as follows: FeO → FeO → FeN → FeC. There was considerable overlap between the different phases, but we were unable to ascertain whether this was due to the experimental setup (side-on heating of a quartz capillary which could lead to thermal gradients) or whether individual particle reactions proceed at different rates. In this paper, we use wide- and small-angle X-ray scattering (wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS)) to demonstrate that the overlapping phases are indeed due to variable reaction rates. While the initial oxide nanoparticles have a small range of diameters, the size range expands considerably and very rapidly during the oxide-nitride transition. This has implications for the isolation of Rietveld-pure FeN, and in an extensive laboratory study, we were indeed unable to isolate phase-pure FeN. However, we made the surprising discovery that Rietveld-pure FeC nanoparticles can be produced at 500 °C with a sufficient furnace dwell time. This is considerably lower than the previous reports of the sol-gel synthesis of FeC nanoparticles.