In situ high-temperature X-ray and neutron powder diffraction study of cation partitioning in synthetic Mg(Fe_0.5Al_0.5)₂O₄ spinel

摘要

The intra-crystalline cation partitioning over T- and M-sites in a synthetic Mg(Fe,Al)₂O₄ spinel sample has been determined as a function of temperature by Rietveld structure refinements from powder diffraction data, combining in situ high-temperature neutron powder diffraction (NPD; POLARIS diffractometer, at ISIS, Rutherford Appleton Laboratory, UK), to determine the Mg and Al occupancy factors, with in situ high-temperature X-ray powder diffraction, to fix the Fe3+ distribution. The results obtained agree with a two-stage reaction, in which an initial exchange between Fe3+ and Mg, the former leaving and the latter entering tetrahedral sites, is successively followed by a rearrangement involving also Al. The measured cation distribution has then been compared and discussed with that calculated by the Maximum Configuration Entropy principle, for which only NPD patterns have been used. The cation partitioning has finally been interpreted in the light of the configuration model of O’Neill and Navrotsky.