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The tie-lines delineating intercrystalline ion-exchange equilibria between MgAl 2 O 4 -MgCr 2 O 4 spinel solid solution and Al 2 O 3 -Cr 2 O 3 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 Al 2 O 3 -Cr 2 O 3 solid solution available in the literature. Activities of Mg 0.5 CrO 2 and Mg 0.5 AlO 2 in the spinel solid solution exhibit strong positive deviations from Raoult's law over most of the composition range. However, activity of Mg 0.5 CrO 2 exhibits mild negative deviation for compositions rich in Mg 0.5 CrO 2 . 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 Al 3ϩ and Cr 3ϩ 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 (Ϯ20) K and X MgCr 2 O 4 ϭ 0.46 (Ϯ0.02), respectively.

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