The temperature dependence of the cation distribution in magnesioferrite (MgFe2O4) from powder XRD structural refinements and Mössbauer spectroscopy

Magnesioferrite (MgFe204 spinel) was synthesized in equilibrium with excess MgO using a flux method. Chemical analysis shows that the MgFe204 is stoichiometric, within analytical uncertainty. Samples were annealed at intervals of 50 °C between 450 and 1250 °C and quenched in H20. The time needed to reach equilibrium at low temperatures (450650 0C) was assessed by monitoring the change of lattice parameter (ao) with time. In addition, the changes in ao at 550-650 °C were reversed, using material annealed at 400 °C for 50 d. Some samples were further characterized by measuring their Neel temperatures, using DSC. The cation distributions in the quenched samples were determined using both powder X-ray diffraction and M6ssbauer spectroscopy. To increase the resolution in the latter, the samples were studied in an applied magnetic field of 4.5 T at 12-171 K. The cation distribution changes smoothly with temperature, with the cation inversion parameter (x) decreasing from 0.90 at 450 °C to 0.72 at 1100 0c. The precision with which x can be determined is ::to.004 from XRD and -::to.O! from the M6ssbauer spectroscopy. The two sets of measurements agree well with each other, the mean difference in x being 0.0056 ::t 0.0102. Thermodynamic modeling shows that the cation distribution can be described with a nonlinear enthalpy of disordering model, with aMg-Fe}+ = 26.6 ::t 0.4 and {1= - 21. 7 ::t 0.3 kJ/g-atom. Incorporation of a small excess (nonconfigurational) entropy term into the model gives a slightly better fit, however. The disordering in MgFe204 as a function of temperature is virtually identical to that found in Fe,04 from thermopower measurements. The present results differ to a greater or lesser extent from most of the previous work on MgFe204. This is most likely attributable to differences in stoichiometry. We also present some results on nonstoichiometric MgFe204, i.e., solutions of MgFe204 and 1'-Fe20 which tend to confirm this hypothesis, at least for some of the earlier studies.