Activities in the spinel solid solution FexMg1-xAl2O4

Activities in the spinel solid solution FexMg1-xAl2O4 saturated with alpha- Al2O3 have been measured for the compositional range 0 < X < 1 between 1100 and 1350 K using a bielectrolyte solid-state galvanic cell, which may be r epresented as Pt, Fe + FexMg1-xAl2O4 + alpha-Al2O3//(Y2O3)ThO2/ (CaO)ZrO2// Fe + FeAl2O4 + alpha-Al2O3, Pt Activities of ferrous and magnesium aluminat es exhibit small negative deviations from Raoult's law. The excess free ene rgy of mixing of the solid solution is a symmetric function of composition and is independent of temperature: Delta G(E) = -1990 X(1 - X J/mol. Theore tical analysis of cation distribution in spinel solid solution also suggest s mild negative deviations from ideality. The lattice parameter varies line arly with composition in samples quenched from 1300 K. Phase relations in t he FeO-MgO-Al2O3 system at 1300 K are deduced from the results of this stud y and auxiliary thermodynamic data from the literature. The calculation dem onstrates the influence of intracrystalline ion exchange equilibrium betwee n nonequivalent crystallographic sites in the spinel structure on intercrys talline ion exchange equilibrium between the monoxide and spinel solid solu tions (tie-lines). The composition dependence of oxygen partial pressure at 1300 K is evaluated for three-phase equilibria involving the solid solutio ns Fe + FexMg1-xAl2O4 + alpha-Al2O3 and Fe + FeyMg1-yO + FexMg1-xAl2O4. Dep endence of X, denoting the composition of the spinel solid solution, on par ameter Y, characterizing the composition of the monoxide solid solution wit h rock salt structure, in phase fields involving the two solid solutions is elucidated. The tie-lines are slightly skewed toward the MgAl2O4 corner.