Thermodynamic stability of non-stoichiometric SrFeO 3-δ : a hybrid DFT study

Abstract

SrFeO3-δ is mixed ionic-electronic conductor with complex magnetic structure which reveals also colossal magnetoresistance effect. This material and its solid solutions are attractive for various spintronic, catalytic and electrochemical applications, including cathodes for solid oxide fuel cells and permeation membranes. Its properties strongly depend on oxygen non-stoichiometry. Ab initio hybrid functional approach was applied here for a study of thermodynamic stability of a series of SrFeO3-δ compositions with several non-stoichiometries δ, ranging from 0 to 0.5 (SrFeO3 - SrFeO2.875 - SrFeO2.75 - SrFeO2.5) as the function of temperature and oxygen pressure. The results obtained by considering Fe as all-electron atom and within the effective core potential technique are compared. Based on our calculations, the phase diagrams were constructed allowing the determination of environmental conditions for the existence of stable phases. It is shown that (within an employed model) only the SrFeO2.5 phase appears to be stable. The stability region for this phase is re-drawn at the contour map of oxygen chemical potential, presented as a function of temperature and oxygen partial pressure. A similar analysis is also performed using experimental Gibbs energies of perovskite formation from the elements. The present modelling strongly suggests a considerable attraction between neutral oxygen vacancies. These vacancies are created during a series of above mentioned SrFeO3-δ mutual transformations accompanied by oxygen release.