Thermodynamics and kinetics of cation ordering in MgAl ₂ O ₄ spinel up to 1600 degrees C from in situ neutron diffraction

The temperature dependence of the cation distribution in synthetic spinel (MgAl 2 O 4 ) was determined using in-situ time-of-flight neutron powder diffraction. Neutron diffraction patterns of stoichiometric MgAl 2 O 4 and slightly non-stoichiometric Mg 0.99 Al 2 O 4 samples were collected under vacuum on heating from room temperature to 1600 C, and the cation distribution was determined directly from site occupancies obtained by Rietveld refinement. The equilibrium non-convergent ordering has been analyzed using both the O'Neill-Navrotsky and Landau thermodynamic models, both of which fit the observed behavior well over the temperature range of the measurements. Fitting the data between 560 C and 1600 C using the O'Neill and Navrotsky (1983) thermodynamic model yields 32.8 0.9 kJ/mol and 4.7 2.0 kJ/mol. The fit to the Landau expression for ordering gives values of T c 445 109 K and c 1.62 0.21. This confirms suggestions that the sign of the coefficient in FeAl 2 O 4 and MgAl 2 O 4 is positive, and opposite to that found in other 2-3 oxide spinels. Non-equilibrium order-disorder behavior below 600 C has been analyzed using the Ginzburg-Landau model, and successfully explains the time-temperature dependent relaxation behavior observed in the inversion parameter. Changing the stoichiometry, even by as little as 1 mol% Mg-deficiency, significantly reduces the degree of order.

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