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

Abstract

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 detailed mechanistic understanding of how oxygen is transported 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 La 2-x Sr x NiO 4+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 Supporting Information

Oxygen transport in Pr nickelates: Elucidation of atomic-scale features

“…Relatively low migration barrier is caused by cooperative motion of polyhedra [30]. Calculated values of the migration barrier for this mechanism were reported to be in the range of 0.73-0.80 eV for La 2 CoO 4+δ , 0.71 eV for La 1.2 Sr 0.8 CoO 4.1 and 0.3-0.7 eV for La 2−x Sr x NiO 4+δ which agree with the oxygen diffusion activation energy values according to the isotope exchange data [30,31,36]. It was reported that the interstitial oxygen anions can be both O 2− and O − with migration of the last one being more energetically favorable [41], and interstitial peroxide anions being more stable than the oxide ones (as demonstrated for La 1.85 Sr 0.15 CuO 4+δ [40]).…”

“…Oxygen transport mechanism in first homologues of R-P phases was modeled by such methods as molecular dynamics (MD) [25][26][27][28][29][30][31][32] and density functional theory (DFT) calculations [23,30,[32][33][34][35][36][37][38][39][40]. The dominating mechanism of oxygen transport in K 2 NiF 4 -like compounds is socalled cooperative mechanism (being also referred to as interstitialcy mechanism [30], cooperative interstitialcy mechanism [31] or interstitialcy-mediated mechanism [36]) as mentioned above involving lattice oxygen of perovskite layers and highly mobile interstitial oxygen of rock salt layers with the second one being able to be accumulated in a large amount [23,30,31,36,38,39,41,42]. The interstitial oxygen anions pass through the Ln 3 triangles to displace the apical oxygen anions of the NiO 6 octahedra, which move through the neighboring Ln 3 triangle to occupy the interstitial places.…”

“…Cooperative mechanism of oxygen migration can also involve oxygen vacancies in apical sites [36,40]. Interstitial oxide anion formation energy is lower in oxidizing conditions, while the vacancy formation energy is lower in reducing conditions [40].…”

“…The other mechanism being referred to as oxide-peroxide mechanism proposed for first R-P homologues considers involving both oxide (O 2− ) and peroxide ((O 2 ) 2− ) anions (which are also can be labelled as "O − ") with migration barrier for peroxide anions being lower compared to that for oxide ones or higher in some cases [36,41,44]. The distance between O atoms in such anions was calculated as 1.5-1.7 Å.…”

Self-Diffusion in Perovskite and Perovskite Related Oxides: Insights from Modelling

“…It should be noted that research on the Ruddlesden-Popper series oxides continues to attract the interest of the research community and although self-diffusion is well characterised theoretically and experimentally, the focus is on proton diffusivity, doping and structural/electronic/magnetic properties [126][127][128][129][130][131][132][133][134][135].…”