Anomalous valence contrast of metal transition in nanocrystalline ferrite

Magnetic and electrical properties of nanocrystalline ferrites depend on the cation distribution, such as the Fe2+ and Fe3+ cations in the spinel structure. In this work, a method based on X-ray anomalous scattering was set up in order to determine the cation distribution. Using an incident beam having an energy close to the iron K edge, the scattering factor of this cation is then modified. Due to a chemical shift of about 5 eV between the K edge of the Fe2+ and Fe3+ cations, the anomalous scattering factors of these cations are slightly different. A contrast between Fe2+ and Fe3+ is observed by X-ray powder diffraction performed at different wavelengths. It implies a relative variation of the diffracted intensity of some hkl lines in the different XRD patterns. The cation distribution is then determined using a Rietveld refinement taking into account the whole set of diffraction patterns. In order to validate the method, some experiments were carried out on the magnetite Fe3O4 as a reference material and on a titanium substituted ferrite. This method seems to be a powerful tool to study such materials, especially for studies in non ambient condition as under controlled atmosphere.