Understanding oxygen diffusion along grain boundaries (GBs) is critical for controlling ionic conductivity in oxide ceramics. GBs are typically thought to enhance ionic transport due to structural disorder and increased free volume. In this study, we report an unexpected anomaly: the Σ3(111) GB in thorium dioxide (ThO2), despite its compact and coherent structure, exhibits significantly higher oxygen ionic conductivity compared to the more open GB (Σ19 as an example). Using atomistic simulations based on a machine learning interatomic potential, we revealed that the high conductivity in the Σ3 GB arises from a collective diffusion mechanism involving highly correlated atomic motion reminiscent of a superionic state. In contrast, the Σ19 GB follows conventional pipe diffusion, consistent with its more open structure. This comparison highlights that enhanced GB conductivity is not simply correlated with free volume, but can occur from specific structural motifs that enable collective transport. These findings provide new guidance for designing GB-engineered oxides with targeted ionic transport properties for energy applications.
@article{arxiv.2507.04652,
title = {Anomalous Ionic Conductivity along the Coherent $\Sigma$3 Grain Boundary in ThO2},
author = {Miaomiao Jin and Jilang Miao and Marat Khafizov and Beihan Chen and Yongfeng Zhang and David H. Hurley},
journal= {arXiv preprint arXiv:2507.04652},
year = {2025}
}