PNNL

Abstract

Fe L2,3-edge XAS and XMCD studies have been used to unravel structural trends in the MgAl2O4–Fe3O4, solid solution where thermodynamic modelling has represented a challenge due to the complex ordering arrangements of the end-members. Partitioning of Fe3+ and Fe2+ between tetrahedral (Td) and octahedral (Oh) sites has been established. In the most Fe-rich samples, despite rapid quenching from a disordered state, Fe2+Td is not present which matches the ordered, inverse spinel nature of end-member magnetite (Mgt) at room-T. However, in intermediate compositions Al and Mg substantially replace Fe and small amounts of Fe2+Td are found, stabilized or trapped by decreasing occurrence of the continuous nearest neighbour Fe – Fe interactions which facilitate charge redistribution by electron transfer. Furthermore, in the composition range ~Mgt0.4-0.9, XAS and XMCD bonding and site occupancy data suggest that nano-scale, magnetite-like Fe clusters are present. By contrast, at the spinel-rich end of the series, Mgt0.17 and Mgt0.23 have a homogeneous long-range distribution of Fe, Mg and Al. These relationships are consistent with the intermediate and Fe-rich samples falling within a wide solvus in this system such that the Fe-clusters occur as proto-nuclei for phases which would exsolve following development of long-range crystalline order during slow cooling. Calculated saturation magnetic moments (Ms) for the Fe-rich samples show excellent agreement with measured values but for the most Mg-rich samples are displaced to slightly higher values; this displacement is due to the presence of abundant Mg and Al disrupting the anti-parallel alignment of electron spins for Fe atoms.