  The development of Ruddlesden-Popper (RP) oxides as oxygen exchange and
transport materials will benefit from mechanistic understanding of how oxygen
transports through these materials. Using Density Functional Theory, we found
there are two distinct oxygen interstitial diffusion mechanisms involving two
different oxygen interstitial species that can be active in La2-xSrxNiO4+delta,
and, we believe, in hyperstoichiometric RP 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
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 and peroxide interstitial.
The stability of the oxide and peroxide, and the competition between the two
oxygen diffusion mechanisms, is 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 -2% to +2%
epitaxial strain. The presented understanding of factors governing interstitial
oxygen diffusion has significant implications for the engineering of RP oxides
in numerous alternative energy technologies.
