Activities and their relation to cation distribution in NiAl<SUB>2</SUB>O<SUB>4</SUB>---MgAl<SUB>2</SUB>O<SUB>4</SUB> spinel solid solutions

The activity of NiAl2O4 in NiAl2O4---MgAl2O4 solid solutions has been measured by using a solid oxide galvanic cell of the type, Pt, Ni + NiAl2O4 + Al2O3(&#945;)/CaO---ZrO2/Ni + NixMg1-xAl2O4 + Al2O3(&#945;). Pt, in the temperature range 750-1150&#176;C. The activities in the spinel solid solutions show negative deviations from Raoult's law. The cation distribution in the solid solutions has been calculated using site preference energies independent of composition for Ni2+, Mg2+, and Al3+ ions obtained from crystal field theory and measured cation disorder in pure NiAl2O4 and MgAl2O4, and assumi g ideal mixing of cations on the tetrahedral and octahedral positions. The calculated values correctly predict the decrease in the fraction, &#945;, of Ni2+ ions on tetrahedral sites for 1&gt;x&gt;0.25, observed by Porta et al. [J. Solid State Chem. 11, 135 (1974)] but do not support their tentative evidence for an increase in a for x &lt; 0.25. The measured excess free energy of mixing can be completely accounted for by using either the calculated or the measured cation distributions. This suggests that the Madelung energy is approximately a linear function of composition in the solid solutions. The composition of NiO---MgO solid solutions in equilibrium with NiAl2O4---MgAl2O4 solid solutions has been calculated from the results and information available in literature