Interplay between chemical strain, defects and ordering in Sr1-xLaxFeO3 materials

Abstract

Different point defect interactions were found to govern the defect chemistry of SrFeO3-δ and La0.6Sr0.4FeO3-δ. Conventional defect structure model based on two reactions – oxygen release by oxide lattice and the charge disproportionation in Fe-sublattice – can be applied successfully to La0.6Sr0.4FeO3-δ. Successful verification of this model using the available data on the oxygen nonstoichiometry gives virtually zero standard entropy and comparatively high standard enthalpy of iron disproportionation (116.54 ± 1.14 kJ/mol). In turn, the chemical strain of La0.6Sr0.4FeO3-δ was predicted successfully using the simple dimensional model, based on the ionic radii formalism and verified defect structure model. For SrFeO3-δ, a reference set of nonstoichiometry data was chosen from among the multitudinous literature data by comparing the calculated and calorimetrically determined oxide's reduction enthalpies. Some aspects of perovskite – brownmillerite phase transition in SrFeO3-δ were discussed and the defect structure model for this oxide was proposed and then verified using the chosen data set. Introduction of the vacancy cluster formation in the defect structure model was shown to be necessary since SrFeO3-δ is highly nonstoichiometric with respect to oxygen and tends to form various ordered structures. As a consequence, chemical expansivity of SrFeO3-δ with respect to the oxygen nonstoichiometry was found to be much more complex than that of La0.6Sr0.4FeO3-δ. According to our findings, unusually high values and the anomalous character of chemical strain of SrFeO3-δ are likely to be attributed to the vacancy cluster formation (i.e. short-range ordering) and some degree of long-range vacancy ordering, respectively.