Influence of Crystal Orientation and Annealing on the Oxygen Diffusion and Surface Exchange of La₂NiO_4+δ

La₂NiO_4+δ is a mixed ionic-electronic conducting material with the 2D K₂NiF₄-type structure garnering much interest as a potential intermediate-temperature solid-oxide fuel cell (IT-SOFC) cathode. The oxygen diffusion along the ab-plane exhibiting the highest oxygen ionic conductivity governs the behavior of the bulk materials in this family of oxides. The oxygen surface exchange processes, however, are not well-understood and large differences in the surface exchange coefficient (k∗) values can be found in the literature for this and other cathode materials. The isotopic exchange depth profiling (IEDP) technique was used in combination with low energy ion scattering (LEIS) measurements on two sets of La₂NiO_4+δ single crystals with precisely cut crystal faces but different thermal histories. For each set of single crystals, the oxygen diffusion and surface exchange coefficient were evaluated for two different orientations in the temperature range of 450-600 °C. The differences in k∗ have been correlated with differences in surface chemistry: surface termination, near-surface rearrangement, and the presence of extrinsic impurities. Finally, the predominant La termination at the immediate outer surface is evidenced, confirming recent results for other Ruddlesden-Popper phases with mixed ionic-electronic conducting properties.