In-situ neutron diffraction study of non-convergent cation ordering in the (Fe3O4) (sub 1-x) (MgAl2O4)xspinel solid solution

Abstract

Non-convergent cation ordering in the (Fe3O4)1-x(MgAl2O4)x solid solution was investigated using in-situ time-of-flight neutron powder diffraction. The approach to equilibrium in a sample with x = 0.75 was observed at 923 K by performing in-situ structure refinements at intervals of 5 min, and the ordering behavior was traced through the time-dependence of the lattice parameter, the cation-oxygen bond lengths, and the cation-site scattering lengths. The data are consistent with a two-stage kinetic process in which relatively rapid exchange of Fe3+ with Mg and Fe2+ between tetrahedral and octahedral sites was followed by slower exchange of Mg with Al. The Fe3+ cations are shown to order onto tetrahedral sites, contrary to the predictions of thermodynamic models for the solid solution.