Structure of magnetite (Fe ₃O ₄) above the Curie temperature: A cation ordering study

A pristine magnetite (Fe ₃O ₄) specimen was studied by means of Neutron Powder Diffraction in the 273-1,073 K temperature range, in order to characterize its structural and magnetic behavior at high temperatures. An accurate analysis of the collected data allowed the understanding of the behavior of the main structural and magnetic features of magnetite as a function of temperature. The magnetic moments of both tetrahedral and octahedral sites were extracted by means of magnetic diffraction up to the Curie temperature (between 773 and 873 K). A change in the thermal expansion coefficient around the Curie temperature together with an increase in the oxygen coordinate value above 700 K can be observed, both features being the result of a change in the thermal expansion of the tetrahedral site. This anomaly is not related to the magnetic transition but can be explained with an intervened cation reordering, as magnetite gradually transforms from a disordered configuration into a partially ordered one. Based on a simple model which takes into account the cation-oxygen bond length, the degree of order as a function of temperature and consequently the enthalpy and entropy of the reordering process were determined. The refined values are ΔH ⁰ = -23. 2(1. 7) kJ mol ^-1 and ΔS ⁰ = -16(2) J K ^-1 mol ^-1. These results are in perfect agreement with values reported in literature (Mack et al. in Solid State Ion 135(1-4):625-630, 2000; Wu and Mason in J Am Ceramic Soc 64(9):520-522, 1981).