Abstract
For the next-generation high temperature microreactors, yttrium dihydride (YH2) is an attractive solid state neutron moderator. Despite a number of recent investigations, the mechanism of hydrogen transport remains poorly understood. Experimental evaluations of diffusivity are inconclusive with large variations in diffusivities and activation energies. In this work, we perform ab initio molecular dynamics (AIMD) simulations on YH2 for temperatures spanning 300 K to 1200 K. Our main finding is that YH2 shows a superionic-like behavior with hydrogen atoms hopping from one native site to another above a characteristic temperature of 800 K. This correlated motion results in quasi-one-dimensional string-like displacements that enable the hydrogen atoms to diffuse rapidly. We confirm that the octahedral sites are mostly unoccupied, although channeling through them is the most favored pathway between lattice hops above 800 K. At the highest temperature of 1200 K, the string relaxation time is merely of the order of a few picoseconds, which indicates a liquid-like diffusive behavior. Based on the formation of spontaneous thermal vacancies, an order-disorder crossover temperature Tα ~ 800 K is established for YH2 with an activation energy of 0.83 eV for hydrogen diffusion in the superionic-like state.
| Original language | English |
|---|---|
| Article number | 18144 |
| Journal | Scientific Reports |
| Volume | 15 |
| Issue number | 1 |
| DOIs | |
| State | Published - Dec 2025 |
Funding
Funding from Idaho National Laboratory LDRD and NUC faculty summer program are gratefully acknowledged (YH and JE). This work is also partly supported by the US Department of Energy (DOE) under the contract DE-AC05-00OR227 (MNC and JY). The authors also thank useful discussions with A. P. Shivprasad and W. P. Scarlett. YH performed the simulations, analyses and prepared the first draft of the manuscript; all the authors contributed to writing the final manuscript. The project was conceived by JE, MNC and JY. Since the manuscript is co-authored by UT-Battelle, LLC, and partially supported by the US DOE, the US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ).