Redox energetics of SrFeO3−δ — a coulometric titration study

The variation of the oxygen non-stoichiometry (3−δ) in SrFeO3−δ with the partial pressure of oxygen was measured by coulometric titration at 1223, 1248 and 1273 K. An electrochemical cell that could be heated and cooled repeatedly without introducing leakages was developed for the experiments. The Gibbs energy, the standard entropy and the partial molar enthalpy and entropy of oxygen of SrFeO3−δ are derived from the measured isotherms and compared with earlier reported data. Solid solution models are presented and used in the analysis of the observed redox properties. It is shown that the redox properties can be interpreted by a non-ideal solution model that includes excess configurational entropy as the only excess term. A reduced structural disorder on the O-sub-lattice is suggested from the formation of preferred structural entities such as FeO6-octahedra, FeO5-square pyramids and FeO4-tetrahedra.