Abstract
Magnesioferrite spinel, MgFe2O4, was synthesized at 900 °C from equimolar amounts of reagentgrade oxides, MgO and Fe2O3, and quenched in air. The structural behavior of magnesioferrite was determined from in situ synchrotron X-ray powder-diffraction data [λ = 0.92225(4) Å] at room pressure and temperatures from 28 to 982 °C on heating and cooling. The a unit-cell parameter increases linearly on heating, but deviates to give a discontinuity at 581 °C. Above 581 °C and on cooling from 982 °C, the a parameter varies linearly. The a parameter at 28 °C before heating [8.39704(5) Å] and after cooling to 47 °C [8.39514(4) Å] 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]2O4}, 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 581 °C, where the sample achieves the maximum ordered state on heating [Xmax = 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 [Tmax = 982 °C, x = 0.769(3)] and reverse along the equilibrium pathway on cooling. At TB, the maximum equilibrium order is frozen in, and maintained to room temperature, where Xmax = 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 MgFe2O4 contains an irreversible exothermic peak at about 550 °C = Trelax 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, Trelax ≈ 581 °C. The TCurie ≈ 360 °C was obtained from TG experiments carried out in a magnetic field.
© 2015 by Walter de Gruyter Berlin/Boston
