Thermodynamic properties of the calcium ferrites CaFe₂O₄ and Ca₂Fe₂O₅

Abstract

The chemical potentials of CaO in the two-phase fields Fe₂O₃ + CaFe₂O₄ and CaFe₂O₄ + Ca₂Fe₂O₅ of the pseudobinary system CaO - Fe₂O₃ have been measured in the temperature range from 975 to 1275 K, relative to pure CaO as the reference state, using solid state galvanic cells incorporating single-crystal CaF₂ as the solid electrolyte. The cell was operated under pure oxygen at ambient pressure. The standard Gibbs energies of formation of calcium ferrites, CaFe₂O₄ and Ca₂Fe₂O₅, were derived from the reversible emfs. The results can be summarized by the following equations:CaO + Fe₂O₃ → CaFe₂O₄;Delta G degrees = - 37,480 + 1.16 T (+/- 250) J/mol 2 CaO + Fe₂O₃ → Ca₂Fe₂O₅;Delta G degrees = - 45, 280 - 13.51 T (+/- 275) J/mol These values are compared with thermodynamic data reported in the literature. The results of this study are in excellent agreement with heat capacity data, and in reasonable agreement with earlier measurements of enthalpy and Gibbs energy of formation, but suggest significant revision of enthalpies of formation of calcium ferrites given in current thermodynamic compilations.