Group led by the University of Tokyo finds that atoms undergo high-speed diffusion along grain boundaries

A group consisting of Distinguished Professor Yuichi Ikuhara of the Advanced Institute for Materials Research at Tohoku University, Professor Naoya Shibata, Project Associate Professor Ryo Ishikawa, and Project Researcher Toshihiro Futazuka, all from the Institute of Engineering Innovation, School of Engineering at the University of Tokyo, together with Professor Katsuyuki Matsunaga and Associate Professor Tatsuya Yokoi from Nagoya University, have elucidated the mechanism by which atoms undergo high-speed diffusion along grain boundaries using atomic-resolution electron microscopy and theoretical calculations (simulations). Their findings were published in Nature Communications.

When trace amounts of dopant elements are introduced into polycrystalline ceramics, various material properties can be enhanced. The dopant elements introduced into such polycrystalline materials preferentially diffuse along grain boundaries and segregate at the boundaries. While macroscopic concentration profile analysis of dopant elements along grain boundaries has been conducted, the formation process of grain boundary segregation during sintering remained unclear. Additionally, studies on grain boundary diffusion during sintering phenomena that employs theoretical calculations had been extremely limited.

The research group successfully conducted direct observation of Hf (hafnium) atoms diffusing along grain boundaries in aluminum oxide (α-Al₂O₃) using a time-resolved atomic-resolution scanning transmission electron microscope (STEM) with atomic resolution. The grain boundary has a structure in which two seven-membered rings and two six-membered rings composed of Al atoms are cyclically arranged. The observed Hf atom diffusion processes were classified into two types: diffusion between Al sites within the grain boundary and diffusion via interstitial sites within the grain boundary. Generally, diffusion via interstitial sites rarely occurs within crystals because it requires very high energy, but it was found that interstitial sites play an important role in high-speed diffusion at grain boundaries.

Next, the Hf atom grain boundary diffusion mechanism was analyzed through theoretical calculations using a supercomputer. The calculations employed a machine learning potential developed by the Matsunaga-Yokoi group at Nagoya University. Using a structure search algorithm called the Simulated Annealing method, the most stable atomic structure of the aluminum oxide grain boundary was determined, and the defect formation energy of Hf atoms at the grain boundary was evaluated. The results suggested that the concentration of Hf atoms and Al vacancies at the grain boundary significantly increases the probability of encounters between Hf atoms and Al vacancies, promoting Hf atom diffusion through the vacancy exchange mechanism.

Finally, Hf atom diffusion pathways and activation energies were evaluated through theoretical calculations. In most areas of the grain boundary, it was shown that the activation energy for Hf atom diffusion via the vacancy exchange mechanism is lower than within the crystal, significantly promoting diffusion (average 1.37 eV, compared with 2 eV within the crystal). However, for interstitial diffusion, pathways with remarkably low activation energy of 0.5 eV were confirmed, involving multiple Al atoms and Al vacancies. It was revealed that in locations where the grain boundary structure is significantly distorted compared with the crystal, the activation energy for diffusion via interstitial sites is low, accelerating grain boundary diffusion.

In this study, the researchers directly observed the diffusion of dopant elements at grain boundaries through analysis that combined advanced electron microscopy and theoretical calculations and clarified that dopant elements undergo high-speed diffusion through exchange mechanisms at vacancy and interstitial sites. Based on these findings, controlling the diffusion behavior of dopant elements at grain boundaries is expected to improve material properties such as ionic conductivity, electron transport, and thermal conductivity. Materials design based on understanding diffusion mechanisms at the atomic level is expected to lead to the development of new materials with high efficiency and high performance.

Journal Information
Publication: Nature Communications
Title: Direct observation of substitutional and interstitial dopant diffusion in oxide grain boundary
DOI: 10.1038/s41467-025-64798-w