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The development of Ruddlesden-Popper (RP) oxides as oxygen exchange andtransport materials will benefit from mechanistic understanding of how oxygentransports through these materials. Using Density Functional Theory, we foundthere are two distinct oxygen interstitial diffusion mechanisms involving twodifferent oxygen interstitial species that can be active in La2-xSrxNiO4+delta,and, we believe, in hyperstoichiometric RP oxides in general. The firstmechanism is the previously proposed interstitialcy-mediated mechanism, whichconsists of diffusing oxide interstitials. The second mechanism is newlydiscovered in this work, and consists of both oxide and peroxide interstitialspecies. This mechanism exhibits a similar or possibly lower migration barrierthan the interstitialcy mechanism for high oxidation states. Which mechanismcontributes to the oxygen interstitial diffusion is the result of the change inrelative stability between the oxide interstitial and peroxide interstitial.The stability of the oxide and peroxide, and the competition between the twooxygen diffusion mechanisms, is sensitive to the overall oxidation state of thesystem. Therefore, the oxygen diffusion mechanism is a function of the materialcomposition, oxygen off-stoichiometry, operating temperature and oxygen partialpressure. We also examined the effect of epitaxial strain on both oxygendiffusion mechanisms, and found that tensile and compressive epitaxial strainof up to 2% had less than 100 meV/(% strain) effects on oxygen interstitialformation, migration, and activation energies, and that total achievableactivation energy reductions are likely less than 100 meV for -2% to +2%epitaxial strain. The presented understanding of factors governing interstitialoxygen diffusion has significant implications for the engineering of RP oxidesin numerous alternative energy technologies.

Factors controlling oxygen interstitial diffusion in the Ruddlesden-Popper oxide La2-xSrxNiO4+delta