Palladium oxide equilibration and the thermodynamic properties of MgAl₂O₄ spinel

The technique of palladium oxide equilibration was used to measure activities of MgO and Al₂O₃ in stoichiometric MgAl₂O₄ spinel at 1150–1400 °C. Activities of MgO range from 0.63 ± 0.03 to 1.00 ± 0.07, and activities of Al₂O₃ range from 0.09 ± 0.01 to 0.05 ± 0.01 (1σ). The activities yield free energies of formation of spinel from the oxides, $ΔGf0$ ranging from −31 kj/mol at 1150 °C to −39 kj/mol at 1400 °C, with a precision of 2–19% (σ). The derived values of $ΔGf0$ are consistent with the equilibrium amount of disorder present in the spinel under experimental conditions because the measured activities reflect equilibration at high temperature. Calorimetric heat contents were corrected for the state of order using a Landau formulation for the equilibrium amount of disorder in spinel as a function of temperature. The corrected heat contents, together with the measured $ΔGf0$⁠, were used to produce a new, self-consistent C_P function that can account for available experimental data. The modeling results are consistent with previous suggestions that short-range ordering may be important in spinel.