We performed muon spin rotation and relaxation (μSR) experiments to investigate the microscopic mechanism behind the high ionic conductivity (σ) exhibited by hydride (H−) ions in lanthanum hydroxide LaH3−2xOx. The μSR spectra observed at 5--300 K in a sample with x≈0.25 consist primarily of two components which are attributed to muons occupying tetrahedral (Tet) and octahedral (Oct) sites common to H−. The spectra also indicate that muons at the Oct sites (MuO) appear nearly stationary in the time scale of μSR (∼10−5 s), whereas those at the Tet sites (MuT) are subject to the fluctuating local fields. The cusp-like peak in the fluctuation rate around 160 K and the decrease in linewidth at higher temperatures probed by MuT suggest that the jump motion of both MuT (via the vacant Oct sites) and surrounding Oct-site H− contributes to spin relaxation and that the fluctuation frequency is widely distributed. These results indicate that the implanted Mu behave as Mu− and that the jump motion of Mu−/H− is restricted by the availability of nearby vacant sites. On the other hand, the activation energy for the jump is estimated to be 0.11(3) eV, which is significantly different from ∼1.3 eV evaluated from the temperature dependence of σ at high temperatures (≳400 K). In our attempt to resolve this discrepancy, we discuss problems inherent in interpreting σ using the Arrhenius equation, and demonstrate that the behavior of H− ions can be better explained as a viscous fluid exhibiting a glass transition.
@article{arxiv.2603.00852,
title = {Relationship between local hydride ion dynamics and ionic conductivity in LaH$_{3-2x}$O$_x$ inferred from muon study},
author = {M. Hiraishi and S. Takeshita and H. Okabe and K. M. Kojima and A. Koda and S. Iimura and K. Fukui and H. Hosono and R. Kadono},
journal= {arXiv preprint arXiv:2603.00852},
year = {2026}
}