Cation ordering in magnesioferrite, MgFe2O4 to 982 °C using in situ synchrotron X-ray powder diffraction

Magnesioferrite spinel, MgFe2O4, was synthesized at 900degreesC from equimolar amounts of reagent-grade oxides, MgO and Fe2O3, and quenched in air. The structural behavior of magnesioferrite was determined from in situ synchrotron X-ray powder-diffraction data [lambda = 0.92225(4) Angstrom] at room pressure and temperatures from 28 to 982degreesC on heating and cooling. The a unit-cell parameter increases linearly on heating, but deviates to give a discontinuity at 581degreesC. Above 581degreesC and on cooling from 982degreesC, the a parameter varies linearly. The a parameter at 28degreesC before heating [8.39704(5) Angstrom] and after cooling to 47degreesC [8.39514(4) Angstrom] is different because the cation order frozen in the structure is not the same. Cation order, analyzed in terms of the inversion parameter, x, {(iv)[Mg1-xFex](vi)[Mgx/2Fe1-x/2](2)O-4}, and the order parameter, Q = 1-(3/2) x, show no change on heating until the temperature is high enough to cause exchange of Mg2+ and Fe3+ cations between the octahedral and tetrahedral sites. This activation barrier is overcome at 581degreesC, where the sample achieves the maximum ordered state on heating [x(max) = 0.867(4)] and begins to move toward equilibrium. This relaxation is toward a more ordered configuration and is a kinetically controlled process. Above 581 'C, the cations continuously disorder along the equilibrium pathway to the maximum temperature studied [T-max = 982degreesC, x = 0.769(3)] and reverse along the equilibrium pathway on cooling. At T-B, the maximum equilibrium order is frozen in, and maintained to room temperature, where X-max = 0.895(4). O'Neill-Navrotsky, Landau, and Ginzburg-Landau models give good descriptions of the ordering process in MgFe2O4. Simultaneous differential scanning calorimetry (DSC) and thermogravimetry (TG) data were obtained using a Netzsch STA 449C simultaneous TG-DSC instrument. The DSC curve for MgFe3O4 contains an irreversible exothermic peak at about 550degreesC = T-relax in the first heating experiment, and the energy change associated with this peak is -162 J/g ( = -32 KJ/mol), and corresponds to cation relaxation. From Rietveld refinements, T-relax approximate to 581degreesC. The T-Curic approximate to 360degreesC was obtained from TG experiments carried out in a magnetic field.