Impact of the structural anisotropy of La2NiO4+δ on on high temperature surface modifications and diffusion of oxygen

La2NiO4+δ was first studied due to its structural similarities with the High Temperature superconductor La2NiO4+δ and more recently due to its promise as a cathode material in Solid Oxide Fuel Cells as well as an oxygen exchange membrane. It crystallizes in the K2NiF4 layered structure and accommodates highly mobile oxygen at its ground state and is therefore overstoichiometric. During this thesis, pure single crystals of La2NiO4+δ were successfully grown using the floating-zone method, subsequently characterized using neutron and Laue Backscattering diffraction and oriented pieces of single crystal with [100] and [001] orientation were prepared. The surface morphology behavior after long term exposure to high temperature in different atmospheres was observed using microscopy techniques because stability at high temperature is required for application purposes and it was discovered a structural change to nickel-rich phases at T>1173 K. The sensibility of the oxygen non-stoichiometry to cooling was studied and subsequently a new 18O-18O exchange apparatus allowing quenching of the samples using liquid nitrogen was developed. Oxygen selfdiffusion was studied using SIMS in the range 673-873K in both [100] and [001] crystallographic directions. The effect of the disorientation of the sample surface on the determination of the slowest diffusion coefficient was discovered and revealed the very strong anisotropy (>5 orders of magnitude difference) between the different diffusion paths. Finally using HTXRD and oxygen release experiments, it was shown that oxygen diffusion from interstitial oxygen starts to be relevant at 550-600 K and a change of behavior is observed around 700 K, corresponding to a possible change in the diffusion mechanism from interstitial to interstitialcy.