Cation partitioning in Mg_0.70Fe_0.23Al_1.97O₄ synthetic spinels by in situ neutron powder diffraction

Neutron powder diffraction experiments in the temperature range 300-1770 K were performed at BENSC, Berlin, Germany, on synthetic Mg_4.70Fe_0.23;Al_1.97O₄. The cation partitioning over the crystallographic tetrahedral and octahedral sites was determined as a function of temperature through joint Rietveld refinements and advanced minimization techniques. The thermal expansion coefficients of the lattice parameter and inter-atomic bond lengths were also obtained from the full-profile structural refinements. The behaviour of the polyhedral bond-lengths, especially the T-O distances, and of the cell constant upon heating, clearly indicate that the interdiffusion of tetrahedral and octahedral Mg/Al cations starts at about 947 K. This result is straightforwardly supported by the direct analysis of the neutron site scattering factors: Fe always retains tetrahedral coordination at all temperatures, and the cation rearrangement is entirely due to Mg and Al intracrystalline diffusion.