On the enthalpic contribution to the redox energetics of SrFeO3−δ

Abstract

The enthalpy of oxidation/reduction of the grossly non-stoichiometric high-temperature phase SrFeO_3−δ, i.e. the enthalpy ofSrFeO_2.50 (perovskite) + ⅙O₂(g) ⇌ SrFeO_2.8333 (perovskite)has been deduced from calorimetric data. The enthalpy of oxidation of vacancy ordered brownmillerite-type SrFeO_2.50 obtained by direct reaction calorimetry combined with extensive heat capacity data (C. Haavik, T. Atake, H. Kawaji and S. Stølen, Phys. Chem. Chem. Phys., 2001, 3, 3863) shows that vacancy ordering in this particular case makes a significant contribution to the measured oxidation enthalpy. While it is often argued that the vacancy ordering in materials like SrFeO_3−δ depends to some degree on the thermal history of the sample investigated, such effects give a negligible contribution to the directly determined enthalpy of oxidation. The redox properties of SrFeO_3−δ are described through the use of a solution model where the presently deduced enthalpy of oxidation and the earlier reported entropy of oxidation are the only input parameters. The present study indicates that the redox properties of complex non-stoichiometric perovskite-type oxides can be rationalized reasonably well through the use of a simple model description. The main parameters of the model are the difference in enthalpy and vibrational entropy of formation between the two limiting compositions of the solid solution (here SrFeO₃ and SrFeO_2.5) and the configurational entropy.