Thermodynamic properties of Fe3O₄-FeV₂O₄ and Fe3O₄-FeCr₂O₄ spinel solid solutions

Abstract

Solid solutions of Fe30₄-FeV₂0₄ and Fe30₄-FeCr₂0₄ were prepared and equilibrated with Pt under controlled streams of CO/CO, gas mixtures at 1673 K. The concentration of Fe in Pt was used to determine the activity of Fe30₄ in the solid solutions. The activity of the second component was calculated by Gibbshhem integration. From these data, the Gibbs energy of mixing was derived for both systems. The experimental results and theoretical values which are determined from calculated cation distribution compare favorably in the case of vanadite solid solutions but not in the case of chromite solid solutions. The difference is attributed to a heat term arising from lattice distortion due to cation size difference. The positive heat of mixing will give rise to a miscibility gap in the system Fe30₄-FeCr₂0₄ at lower temperatures.