We performed muon spin rotation and relaxation ($\mu \mathrm{SR}$) experiments to investigate the microscopic mechanism behind the high ionic conductivity ($\sigma$) exhibited by hydride (${\mathrm{H}}^{-}$) ions in lanthanum hydroxide ${\mathrm{LaH}}_{3-2x}{\mathrm{O}}_x$. The $\mu \mathrm{SR}$ spectra observed at 5–300 K in a sample with $x\approx 0.25$ consist primarily of two components, which are attributed to muons occupying tetrahedral (Tet) and octahedral (Oct) sites common to ${\mathrm{H}}^{-}$. The spectra also indicate that muons at the Oct sites (${\mathrm{Mu}}_{\mathrm{O}}$) appear nearly stationary in the timescale of $\mu \mathrm{SR}$ ($\approx {10}^{-5}\mathrm{s}$), whereas those at the Tet sites (${\mathrm{Mu}}_{\mathrm{T}}$) 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 ${\mathrm{Mu}}_{\mathrm{T}}$ suggest that the jump motion of both ${\mathrm{Mu}}_{\mathrm{T}}$ (via the vacant Oct sites) and surrounding Oct-site ${\mathrm{H}}^{-}$ contributes to spin relaxation and to a widely distributed fluctuation frequency. These results indicate that the implanted Mu behave as ${\mathrm{Mu}}^{-}$ and that the jump motion of ${\mathrm{Mu}}^{-}/{\mathrm{H}}^{-}$ 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 $\approx 1.3\mathrm{eV}$ evaluated from the temperature dependence of $\sigma$ at high temperatures ($\gtrsim 400\mathrm{K}$). In our attempt to resolve this discrepancy, we discuss problems inherent in interpreting $\sigma$ using the Arrhenius equation, and demonstrate that the behavior of ${\mathrm{H}}^{-}$ ions can be better explained as a viscous fluid exhibiting a glass transition.

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• Glass transition
• Hydrogen storage
• Ionic conductivity
• Hydrides
• Differential scanning calorimetry
• Muon spin relaxation & rotation