Palladium oxide equilibration and the thermodynamic properties of MgAl_2O_4 spinel

Abstract

The technique of palladium oxide equilibration was used to measure activities of MgO and A1_2O_3 in stoichiometric MgAl_2O_4 spinel at 1150-1400 °C. Activities of MgO range from 0.63 ± 0.03 to 1.00 ± 0.07, and activities of Al_2O_3 range from 0.09 ± 0.01 to 0.05 ± 0.01 (1σ). The activities yield free energies of formation of spinel from the oxides ΔG^0_f,ranging from -31 kJ/mol at 1150 °C to -39 kJ/mol at 1400 °C, with a precision of 2-19% (1σ). The derived values of ΔG^0_f 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 ΔG^0_f, 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.