Determination of Chemical Tracer Diffusion Coefficients for the La- and Ni-site in La2NiO4+δ Studied by SIMS


  • Presented at the 17th Conference on Solid State Ionics, June 28–July 3, 2009, Toronto, Canada (oral presentation at T3C-Ionic Transport Session).

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Chemical (impurity) tracer diffusion of Pr, Nd, and Co into polycrystalline La2NiO4+δ was done at 950°C–1350°C in air, argon, and intermediate pO2 (5.5 × 10−3 atm O2), and diffusion coefficients were extracted from depth profiles determined by Secondary Ion Mass Spectrometry (SIMS). The Pr and Nd profiles have only one broad region, corresponding to bulk diffusion, whereas the Co tracer depth profile has two distinct regions with different slopes, where the outer shallow region represents bulk diffusion and the inner region with deep penetration depths represents grain-boundary diffusion. It is thus concluded that the diffusivity on the Ni-site is enhanced by grain-boundary diffusion. The bulk diffusion was evaluated using the solution of Fick's second law for thin-film source, and the grain-boundary diffusion was evaluated according to Whipple-Le Claire's equation. The average apparent activation energies for Pr and Nd bulk diffusion are 165 ± 15 kJ/mol, for Co bulk diffusion 295 ± 15 kJ/mol, and for Co grain-boundary diffusion 380 ± 20 kJ/mol. Qualitatively, the diffusivities and activation energies follow levels and trends in agreement with those from other experimental techniques. The apparent lack of—in fact reverse—correlation between activation energy and level of diffusivity is discussed in terms of a possibility that the faster species (Ni) reach equilibrium defect concentrations while the slower (La) is in effect frozen in.