Factors Controlling Oxygen Interstitial Diffusion in the Ruddlesden–Popper Oxide La_2–xSr_xNiO_4+δ

The development of Ruddlesden–Popper oxides as oxygen exchange and transport materials for applications such as solid oxide fuel cells, oxygen separation membranes, and chemical looping will benefit from a detailed mechanistic understanding of how oxygen is transported through these materials. Using density functional theory, we found that there are two distinct oxygen interstitial diffusion mechanisms involving two different oxygen interstitial species that can be active in La2–xSrxNiO4+δ and, we believe, in hyperstoichiometric Ruddlesden–Popper oxides in general. The first mechanism is the previously proposed interstitialcy-mediated mechanism, which consists of diffusing oxide interstitials. The second mechanism is newly discovered in this work and consists of both oxide and peroxide interstitial diffusing species. This mechanism exhibits a similar or possibly lower migration barrier than the interstitialcy mechanism for high oxidation states. Which mechanism contributes to the oxygen interstitial diffusion is the result of the change in relative stability between the oxide interstitial (2– charge) and peroxide interstitial (1– charge), which directly affects the migration barriers for these two different mechanisms. The stability of the oxide and peroxide and therefore the competition between the two oxygen diffusion mechanisms is highly sensitive to the overall oxidation state of the system. Therefore, the oxygen diffusion mechanism is a function of the material composition, oxygen off-stoichiometry, operating temperature, and oxygen partial pressure. We also examined the effect of epitaxial strain on both oxygen diffusion mechanisms and found that tensile and compressive epitaxial strain of up to 2% had less than 100 meV/(% strain) effects on oxygen interstitial formation, migration, and activation energies and that total achievable activation energy reductions are likely less than 100 meV for up to ±2% epitaxial strain. The presented understanding of factors governing interstitial oxygen diffusion potentially has significant implications for the engineering of Ruddlesden–Popper oxides in numerous alternative energy technologies.