Oxygen vacancy formation energies in Sr-doped complex perovskites: ab initio thermodynamic study

摘要

La1-xSrxCo0.25Fe0.75O3-delta is known as one of the best cathode materialsfor permeation membranes and solid oxide fuel cells. Optimization of itschemical composition is a challenging problem. One of the key properties isconcentration of oxygen vacancies, controlled by their formation energies. Abinitio calculations were employed in order to study the formation of oxygenvacancies in La1-xSrxCo0.25Fe0.75O3-delta perovskites by varying the Sr contentfrom x = 12.5% to 50%. The formation energies were obtained for differentstoichiometries as functions of temperature and oxygen partial pressure. Wehave shown that the phonon contribution to the free formation energy becomesincreasingly important in La1-xSrxCo0.25Fe0.75O3-delta not only with risingtemperature but also with rising Sr content. We have shown that the formationenergies are decreased significantly with increasing Sr content due to twoeffects: charge compensation of Sr2+ ions and phonon contribution. We havesuggested a simple explanation to increasing role of phonons for the oxygenvacancies formation energies on the basis of phonon mode changes in comparisonto defect-free materials. A careful analysis of the experimental results fromthe literature is also presented.