Visualizing local fast ionic conduction pathways in nanocrystalline lanthanum manganite by isotope exchange-atom probe tomography

Abstract

The call for material systems with enhanced mass transport properties is central in the development of next-generation fuel cells, batteries and solid state energy devices in general. While two-dimensional doping by artificial heterostructuring or nanoscaling has shown great potential for overcoming kinetic limitations of ion diffusion, the length scale of interface effects requires the development of advanced tools for capturing and quantifying local phenomena in greater detail. In the present paper, an in-depth study of grain boundary oxygen conduction in Sr-doped lanthanum manganite films is presented by means of novel isotope-exchange atom probe tomography. Local pathways for fast mass transport are directly mapped by two-dimensional reconstructions and line profiles of the oxygen isotope concentration. Accurate finite element modelling is employed to retrieve the local kinetic parameters, highlighting an enhancement of two orders of magnitude for both the diffusivity and surface exchange rate with respect to the bulk ( cm² s⁻¹ and cm s⁻¹, respectively, for grain boundaries at 550 °C). Co-acquired reconstruction of the cationic distribution reveals strong inhomogeneities (dopant de-mixing) across the grain boundaries and in the sub-surface region, leading to local Sr accumulation. The findings provide unequivocal quantitative assessment of fast grain boundary oxygen diffusion in lanthanum strontium manganite, giving further insights into local stoichiometry deviations and promoting isotope exchange-atom probe tomography as a powerful tool for the study of local interface effects with high resolution. Different models for the explanation of the phenomena are critically discussed on the basis of the experimental findings.