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

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 [ = 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, {[Mg1–xFex][Mgx/2Fe1–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 Mg and Fe 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 confi guration 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 fi rst heating experiment, and the energy change associated with this peak is –162 J/g (= –32 KJ/mol), and corresponds to cation relaxation. From Rietveld refi nements, Trelax 581 °C. The TCurie 360 °C was obtained from TG experiments carried out in a magnetic fi eld. * E-mail: [email protected] Cubic spinels have the general formula AB2O4. The A and B cation charges may be either +2 and +3 (e.g., magnesioferrite, MgFe2O4), or +4 and +2 (e.g., qandilite, TiMg2O4). In “normal” spinels, the A cation occupies the tetrahedral site and the B cation occupies the octahedral site. In fully “inverse” spinels, the tetrahedral (IV) site contains only B cations and the octahedral (VI) site contains an equal number of A and B cations, so the octahedral site is disordered. Any intermediate spinel may be expressed as a mix of the normal and inverse end-members, with general formula: [A1–xBx][Ax/2B1–x/2]2O4, where the variable x is referred to as the “inversion parameter”. This x is the fraction of B cations at the tetrahedral site. In normal spinels x = 0, and in inverse spinels x = 1. A value of x = 2/3 corresponds to a completely random distribution of A and B cations. Alternatively, an order parameter, Q, is used to express the degree of order (see Harrison et al. 1998), and varies from Q = 1 for a completely ordered normal spinel, to Q = 0 (where x = 2/3) for a random arrangement of cations, to Q = –0.5 in inverse spinel. The relationship between Q and x is: Q = 1 – (3/2) x. MgFe2O4 is partly inverse and partly normal and is, therefore, one of the most interesting ferrite spinels (Paladino 1960). ANTAO ET AL.: IN SITU CATION ORDERING IN MGFE2O4 220 Spinels tend to be completely disordered as the temperature is increased (e.g., O Neill et al. 1992; Faller and Birchenall 1970; Mozzi and Palladino 1963). The order-disorder process in spinels is termed “non-convergent” because there is no symmetry change upon disordering. A completely random distribution would occur at infi nite temperature, and is approached asymptotically with increasing temperature. However, some spinels melt before disorder is complete (Faller and Birchenall 1970). Interest in spinels arises from the ability of two different cations to disorder over two separate cation sites. This disorder phenomenon is called substitutional disorder, and is exhibited by numerous rock-forming minerals (e.g., Hazen and Navrotsky 1996). Samples of MgFe2O4 used in previous studies have different stoichiometries, therefore, the results may not be comparable (e.g., Bacon and Roberts 1953; Mozzi and Paladino 1963; Allen 1966; Walters and Wirtz 1971; Faller and Birchenall 1970; Nell et al. 1989; O Neill et al. 1992; Harrison and Putnis 1999). Departure from stoichiometry in MgFe2O4 results from: (1) substitution of Fe for Mg causing solid solution toward Fe3O4; (2) solid solution toward -Fe2O3 (maghemite); and (3) excess MgO (O Neill et al. 1992). To synthesize stoichiometric MgFe2O4, the temperature should be less than 1000 °C, and the presence of excess MgO avoids the solid-solution problem. The synthesis temperature in this study was 900 °C for magnesioferrite. An X-ray diffraction (XRD) pattern for our sample showed that it was crystalline, although O Neill et al. (1992) stated that a synthesis temperature below 950 °C would give a non-crystalline product. Studies of magnesioferrite quenched from various temperatures are available (e.g., Harrison and Putnis 1999; O Neill et al. 1992; Allen 1966; Walters and Wirtz 1971; Faller and Birchenall 1970). Quenched samples may not be representative of the cation order at the particular annealing temperature because the quenching rate may not be rapid enough to preserve the cation distribution (O Neill et al. 1992). Therefore, in situ measurements at high temperatures are more reliable than measurements on quenched samples. This study determines the cation order in magnesioferrite, MgFe2O4, at room pressure and from 28 to 982 °C on heating and cooling. These results are used to compare the thermodynamic models for cation ordering; namely the O Neill and Navrotsky (1983), Landau (Carpenter et al. 1994; Carpenter and Salje 1994), and the Ginzburg-Landau (Carpenter and Salje 1989; and Salje 1988) models. Some recent magnesioferrite data are also included for comparison (Levy et al. 2004). EXPERIMENTAL METHODS