Analysis of atomic structure, magnetic ordering, and oxygen diffusion in oxygen deficient Sr3Fe2O7−δ perovskite: Toward rational catalysts design

Mechanistic analysis of oxygen transfer in an atomic-scale structure is important for various applications, such as the three-way catalysts for treating ${\mathrm{NO}}_x$ in automotive exhaust and solid oxide fuel cell electrolyte materials. It has been shown that the $n=2$ Ruddlesden-Popper phases ${\mathrm{Sr}}₃{\mathrm{Fe}}₂{\mathrm{O}}_{7.0\ensuremath{-}\ensuremath{\delta}}$ have superior oxygen storage capacity compared to the conventional ${\mathrm{CeO}}₂\text{\ensuremath{-}}{\mathrm{ZrO}}₂$. Atomic structure, magnetic ordering, charged defects arising due to oxygen vacancies, and the effect of lattice parameters on the ionic transport in ${\mathrm{Sr}}₃{\mathrm{Fe}}₂{\mathrm{O}}_{7.0\ensuremath{-}\ensuremath{\delta}}$ within the oxygen content of 5.75 to 7.0 were investigated by using $\mathrm{DFT}+U$ calculations. In the case of ${\mathrm{Sr}}₃{\mathrm{Fe}}₂{\mathrm{O}}_{6.0\ensuremath{-}7.0}$, the oxidation reaction energies were found to be almost independent of the initial oxygen content. We investigated the activation energies of oxygen diffusion by using the equilibrium and compressively strained lattice parameters for ${\mathrm{Sr}}₃{\mathrm{Fe}}₂{\mathrm{O}}_6.5$. The activation energies were found to be nearly proportional to the volume of the unit cell; namely, as the volume of the unit cell increases, the activation energy decreases. It was found that in the oxygen diffusion process the activation energy increases as the bond length between iron and oxygen decreases and the bond length is minimum in the transition state, indicating that the iron-oxygen repulsive interaction is the origin of the activation energy. Moreover, in addition to the dominant migration pathway in ${\mathrm{Sr}}₃{\mathrm{Fe}}₂{\mathrm{O}}_{6.0\ensuremath{-}7.0}$, a new migration pathway in ${\mathrm{Sr}}₃{\mathrm{Fe}}₂{\mathrm{O}}_{5.75\ensuremath{-}6.0}$ was found.