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

TL;DR

This study uses Density Functional Theory to identify two distinct oxygen interstitial diffusion mechanisms in La2-xSrxNiO4+delta, revealing how material composition, oxidation state, and strain influence oxygen transport in Ruddlesden-Popper oxides.

Contribution

It uncovers a new peroxide interstitial diffusion mechanism and analyzes how oxidation state and strain affect oxygen diffusion in RP oxides.

Findings

Two diffusion mechanisms identified: interstitialcy and peroxide-mediated.

Peroxide mechanism can have lower migration barriers at high oxidation states.

Epitaxial strain has minimal impact on oxygen diffusion energies.

Abstract

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.