Ionic transport in solid oxides lays the fundamentals of solid-state electrochemical energy and sensor devices. While ionic transport in polycrystalline oxides has been studied for decades, most studies are reliant upon macro-scale observations where direct revelation of localized transport behavior is not feasible. In this presentation, we report a direct quasi in situ observation of oxygen ion transport in SrTiO₃ (STO) and Y₂O₃-stabilized ZrO₂ (YSZ) by leveraging scanning probe microscopy variants.

First, a cluster of oxygen vacancies formed by an electrical bias at a tip-sample contact. Then, we exposed the sample to an elevated temperature to allow a facilitated ionic diffusion. The resultant lateral distribution of oxygen vacancies was obtained by Kelvin probe force microscopy (KPFM). Depth-wise distribution of oxygen vacancies were acquired by a combination of in situ etching with a diamond-coated probe and KPFM scans, which enabled an observation of ionic charge transport in a highly localized fashion in both lateral and depth-wise directions.

From 3D spatial charge imaging, the activation energies of surface ionic diffusion in the lateral direction (E_surf) and those of bulk diffusion in the depth-wise direction (E_bulk) were quantified; for YSZ, E_surf = 0.34 eV and E_bulk = 1.13 eV, and for STO, E_surf = 0.50 eV and E_bulk = 1.30 eV. It was also found that the surface ion diffusivity of YSZ (1.68 × 10^-8 cm²/s) is more than 3 orders of magnitude higher than that of STO (4.86 × 10^-12 cm²/s), and that bulk diffusivity of YSZ (1.70 × 10^-11 cm²/s) is more than 7 orders of magnitude higher than that of STO (7.61 × 10^-19 cm²/s). To the best of our knowledge, this is the first study where the ionic diffusivities along the surface and into the bulk are separately quantified by direct localized observations of 3D distribution of ionic species, as opposed to a bulk measurement from which diffusivities were indirectly deduced.

The authors acknowledge the support by National Science Foundation (DMR 1753383).