Anisotropic ionic transport properties in La2NiO4+δ single crystals

The anisotropy of anionic conductivity in a mixed conductor oxide (La2NiO4+δ) with the 2D K2NiF4-type structure was measured on a high-quality single crystal. We report measurements of the oxygen diffusivity parallel and perpendicular to the [001] direction, from 450 to 900 °C, using the technique combining isotopic 16O/18O exchange and secondary ion mass spectroscopy (SIMS) spectroscopy. The diffusion is about one to two orders of magnitude higher in the (a,b) plane when compared to the perpendicular direction. The activation energy of the diffusion measured in the (a,b) plane is close to the one (Ea∼0.9 eV) measured in the ceramics and in agreement, as expected, with a mechanism involving the interstitial oxygen Oi2− as major diffusing species. This prevailing mechanism may explain, at least partly, the good efficiency of such materials when used as cathodes in solid oxide fuel cell (SOFC) devices. Although the diffusivity parallel to the [001] axis is small, its activation energy is about four times weaker (Ea∼0.25 eV) than the one observed in the (a,b) plane. This result is in agreement with a non-stoichiometry model involving anti-phase boundaries in such materials, which induce the stabilization of oxygen vacancies in the NiO2 layers. The corresponding proposed mechanism involves the transfer of smaller Oi− species, with a low concentration but a high mobility. Furthermore, electronic conduction seems to play a key role in this mechanism. Finally, we measured a low anisotropy for the values of the surface exchange coefficients.