Low temperature diffusion and oxygen stoichiometry in lanthanum nickelate

The tracer diffusion (D*) and surface exchange coefficients (k*) of polycrystalline lanthanum nickelate, La2NiO4 + δ (LNO), were measured over the temperature range 350 °C–700 °C using isotopic exchange and Secondary Ion Mass Spectrometry (SIMS). Diffusion profiles were obtained at low temperatures after relatively short anneal times. The polycrystalline material exhibits low activation energies for oxygen diffusion and surface exchange, which were calculated to be 0.54 eV and 0.63 eV, respectively. Oxygen excess was determined by iodometric titration after oxygen isotopic exchange, allowing for the calculation of the oxygen interstitial concentration, [Oi″]. At temperatures below 450 °C, [Oi″] increases with decreasing temperature, resulting in the low activation energy for diffusion in this temperature regime and supporting the theory of diffusion via oxygen interstitial species. The oxygen interstitial diffusion coefficient (Di) is calculated and is found to have an activation energy of 0.63 eV over the temperature range 350 °C–550 °C and to be approximately an order of magnitude greater compared with D*. Values calculated for Di are of the same order of magnitude as values for vacancy diffusion coefficients (Dv) in perovskites.