Activities and their relation to cation distribution in NiAl 2O 4MgAl 2O 4 spinel solid solutions

Abstract

The activity of NiAl 2O 4 in NiAl 2O 4MgAl 2O 4 solid solutions has been measured by using a solid oxide galvanic cell of the type, Pt, Ni + NiAl 2O 4 + Al 2O 3(α)/CaOZrO 2/Ni + Ni x Mg 1− x Al 2O 4 + Al 2O 3(α). Pt, in the temperature range 750–1150°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 Ni 2+, Mg 2+, and Al 3+ ions obtained from crystal field theory and measured cation disorder in pure NiAl 2O 4 and MgAl 2O 4, and assumi g ideal mixing of cations on the tetrahedral and octahedral positions. The calculated values correctly predict the decrease in the fraction, α, of Ni 2+ ions on tetrahedral sites for 1> x>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 α for x < 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 NiAl 2O 4MgAl 2O 4 solid solutions has been calculated from the results and information available in literature.