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

The enthalpy of oxidation/reduction of the grossly non-stoichiometric high-temperature phase SrFeO3-delta, i.e. the enthalpy of SrFeO2.50 (perovskite) + 1/6 O-2 (g) reversible arrow SrFeO2.8333 (perovskite) has been deduced from calorimetric data. The enthalpy of oxidation of vacancy ordered brownmillerite-type SrFeO2.50 obtained by direct reaction calorimetry combined with extensive heat capacity data (C. Haavik, T. Atake, H. Kawaji and S. Stolen, 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 SrFeO3-delta 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 SrFeO3-delta 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 SrFeO2.5) and the configurational entropy.