Cation distribution in a titanium ferrite Fe2.75Ti0.25O4 measured by in-situ anomalous powder diffraction using Rietveld refinement

摘要

Many ferrites contain different cations with various valence states and location in the spinel structure. In compounds such as these, only a combination of different techniques such as Mossbauer spectroscopy, IR analysis, and thermogravimetry allows the distribution of cations to be obtained. For very complicated distributions, the mathematical decomposition of derivative thermogravimetric curves (DTG) leading to quantitative distribution is uncertain. In this paper, me present an alternative technique based on resonant diffraction, The anomalous scattering of each cation in the crystalline material is used to determine its amount and position by Rietveld refinement. Since the energy for such an anomalous phenomenon is different for each cation, this technique should lead to the cation distribution whatever its complexity. Such a method in which the wavelength has to be varied with a great accuracy requires a synchrotron radiation source. The method has been tested in the simple case of titanium ferrites where the valence and location of the Fe cations can be adjusted through appropriate thermal treatments in reducing or oxidizing conditions. The aim of this paper is to validate this new method by comparing the cation distribution so obtained with that deduced from thermogravimetry experiments. In particular, we focus on its ability to distinguish between the different degrees of oxidation of the iron cations, Three different diffraction patterns have been recorded using the high-resolution goniometer on the BM02 Beam line at ESRF at the following energies: 7.105 keV, below the adsorption edges of both Fe2+ and Fe3+ cations; 7.120 keV, above the absorption edge for Fe2+ cations but below the Fe3+ edge and 7.135 keV, above the absorption edges of the Fe2+ and Fe3+ cations. (C) 1998 Academic Press. [References: 20]