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. A study was made of measurements of the O diffusivity parallel and perpendicular to the [001] direction, from 450 to 900C, using a technique combining isotopic 16O/18O exchange and secondary ion mass spectroscopy. The diffusion was some 1 to 2 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 was close to that (about 0.9eV) measured in the ceramics and was in agreement with a mechanism that involved interstitial oxygen, Oi2−, as a major diffusing species. Although the diffusivity parallel to the [001] axis was small, its activation energy was about 4 times weaker (about 0.25eV) than that observed in the (a,b) plane. This result was in agreement with a non-stoichiometry model which involved antiphase boundaries in such materials, which induced stabilization of O vacancies in the NiO2 layers. The corresponding proposed mechanism involved the transfer of smaller Oi− species with a low concentration but a high mobility. Electronic conduction seemed to play a key role in this mechanism.

Anisotropic Ionic Transport Properties in La2NiO4+δ Single Crystals. J.M.Bassat, P.Odier, A.Villesuzanne, C.Marin, M.Pouchard: Solid State Ionics, 2004, 167[3-4], 341-7