Gibbs free energies of formation of ZnAl₂O₄ and ZnCr₂O₄

The standard free energies of formation of zinc aluminate and chromite were determined by measuring the oxygen potential over a solid CuZn alloy, containing 10 at.−% Zn, in equilibrium with ZnO, ZnAl₂O₄+Al₂O₃(χ) and ZnCr₂O₄+Cr₂O₃, in the temperature range 700–900°C. The oxygen potential was monitored by means of a solid oxide galvanic cell in which a Y₂O₃ThO₂ pellet was sandwiched between a CaOZrO₂ crucible and tube. The temperature dependence of the free energies of formation of the interoxidic compounds can be represented by the equations, Zn)(wurtzite)+Al₂O₃(α)→ZnAl₂O₄(sp) ΔG°=–10,750+1.57T(±150)cal ZnO(wurtzite)+Cr₂O₃(s)→ZnCr₂O₄(sp) ΔG°=–15,000+2.05T(±150)cal The heat of formation of the spinels calculated from the measurements by the “Second Law method” is found to be in good agreement with calorimetrically determined values. Using an empirical correlation for the entropy of formation of cubic spinel phases from oxides with rock-salt and corundum structures and the measured high temperature cation distribution in ZnAl₂O₄, the entropy of transformation of ZnO from wurtzite to rock-salt structure is evaluated