• journal_title：American Mineralogist
• Contributor：Sytle M. Antao ; Ishmael Hassan ; John B. Parise
• Publisher：Mineralogical Society of America
• Date：2005-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2005.1559
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：90
• issue：1
• firstpage：219
• section：Articles

Magnesioferrite spinel, MgFe₂O₄, was synthesized at 900 °C 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 [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[Mg_1–xFe_x]^vi[Mg_x/2Fe_1–x/2]₂ O₄}, 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 [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 = 982 °C, 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 MgFe₂O₄ contains an irreversible exothermic peak at about 550 °C = 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 ≈ 581 °C. The T_Curie ≈ 360 °C was obtained from TG experiments carried out in a magnetic field.