Palladium oxide equilibration and the thermodynamic properties of MgAl2O4 spinel

The technique of palladium oxide equilibration was used to measure activities of MgO and A1203 in stoichiometric MgA120. spinel at 1150-1400 0c. Activities of MgO range from 0.63 :t 0.03 to 1.00 :t 0.07, and activities of Al203 range from 0.09 :t 0.01 to 0.05 :t 0.01 (10). The activities yield free energies of formation of spinel from the oxides tJ.G~,ranging from -31 kJ/mol at 1150 °C to -39 kJ/mol at 1400 °C, with a precision of 2-19% (10). The derived values of tJ.G~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 tJ.G~,were used to produce a new, self-consistent Cp function that can account for ~vailable experimental data. The modeling results are consistent with previous suggestIOns that short-range ordering may be important in spinel.