Equilibria involving the reciprocal spinel solid solution (MgxFe1-x)(AlyCr1-Y)(2)O-4: Modeling and experiment

Developed in this article is a model for calculating cation distribution and activities in the reciprocal spinel solid solution (MgXFe1–X)(AlYCr1–Y)2O4 based on octahedral site preference energies of cations independent of composition and temperature, random distribution of ions on tetrahedral and octahedral sites, entropy of randomization of Jahn–Teller distortions associated with Fe2+ ions on the tetrahedral site, and the standard Gibbs energies of formation of the four pure spinel compounds. Enthalpy of mixing of this reciprocal solid solution caused by the large difference of ionic radii of Al3+ and Cr3+ present on the octahedral site was modeled based on experimental data on the binary systems. The tie-line compositions corresponding to the equilibria between the spinel solid solution and the sesquioxide solid solution (AlZCr1–Z)2O3 with corundum structure were computed. Values for activities in the corundum solid solution were taken from the literature. The oxygen potential corresponding to the three-phase equilibrium involving metallic iron, the spinel solid solution, and corundum solid solution was computed as a function of composition of the spinel solid solution. The computed results were verified by measurements on nine compositions inside the square representing the reciprocal system. The compositions of coexisting solid solutions were determined by electron-probe microanalysis (EPMA) and lattice parameter measurement using X-ray diffraction (XRD). The activities of FeAl2O4 and FeCr2O4 and oxygen potentials for three-phase equilibria were measured using two independent solid-state cells incorporating a bielectrolyte chain. Both cells gave consistent results within experimental error.The experimental results are in excellent agreement with the computed results, thus validating the model for the reciprocal spinel solid solution