Direct observation of substitutional and interstitial dopant diffusion in oxide grain boundary

Grain boundaries (GBs) serve as fast diffusion paths for dopant atoms, and the segregated dopants can significantly alter the materials' properties. However, the exact mechanism of fast dopant diffusion along the GBs, particularly at atomic scale, is still unclear. Here we show direct observation of preferential GB diffusion of Hf dopant atoms along the Σ31 symmetric tilt GB in α-Al₂O₃, using time-resolved atomic-resolution scanning transmission electron microscopy and statistical tracking of Hf atom locations. Molecular dynamics simulations incorporating artificial neural network interatomic potentials reveal that Hf atoms preferentially diffuse along the GB by exchanging with co-segregated Al vacancies at the GB. Moreover, we demonstrate that GB interstitial diffusion can greatly enhance the diffusivity of Hf atoms along the GB, where shuffle motion plays a key role in lowering the activation energies for GB diffusion.