Spinel-corundum equilibria and activities in the system MgO-Al2O3-Cr2O3 at 1473 K

The tie-lines delineating intercrystalline ion-exchange equilibria between MgAl2O4-MgCr2O4 spinel solid solution and Al2O3-CP2O3 solid solution with corundum structure have been determined at 1473 K by electron microprobe and X-ray diffraction (XRD) analysis of equilibrated phases. The tie-lines are skewed to the solid solution 0.7MgAl(2)O(4)-0.3MgCr(2)O(4). The lattice parameters and molar volumes of both the solid solution series exhibit positive deviations from Vegard's and Retger's laws, respectively. Activities in the spinel solid solution are derived from the tie-line information and thermodynamic data on Al2O3-Cr2O3 solid solution available in the literature. Activities of Mg0.5CrO2 and Mg0.5AlO2 in the spinel solid solution exhibit strong positive deviations from Raoult's law over most of the composition range, However, activity of Mg0.5CrO2 exhibits mild negative deviation for compositions rich in Mg0.5CrO2. The activity-composition relationship in the spinel solid solution is analyzed in terms of the intracrystalline exchange of cations between the tetrahedral and octahedral sites of the spinel structure. The intracrystalline ion exchange is governed by site preference energies of the cations, The difference between the Gibbs energy of mixing calculated using the cation mixing model and the experimental data is taken as a measure of the strain contribution arising from the difference in the radii of Al3+ and Cr3+ ions. The large positive strain enthalpy suggests the onset of immiscibility in the spinel solid solution at low temperatures. The computed critical temperature and composition for phase separation are 802 (+/-20) K and X-MgCr2O4 = 0.46 (+/-0.02), respectivel