Thermodynamic properties of Fe₃O₄-FeAl₂O₄ spinel solid solutions

Abstract

The Gibbs energy of mixing for the system Fe₃O₄-FeAl₂O₄ was determined at 1573 K using a gas-metal-oxide equilibration technique. Oxide solid solution samples were equilibrated with Pt foils under controlled CO+CO₂ gas streams. The equilibrium iron concentration in the foil was determined by chemical analysis. The cation distribution between tetrahedral and octahedral sites in the spinel crystal can be calculated from site-preference energies and used as an alternate method of determining some thermodynamic properties, including the Gibbs energy of mixing. The solvus occurring at low temperatures in the system Fe₃C₄-FeAl₂C₄ was used to derive the effect of lattice distortion due to cation size difference on the enthalpy of mixing and to obtain a better approximation to the measured thermodynamic quantities.