Relationship between local hydride ion dynamics and ionic conductivity in LaH$_{3-2x}$O$_x$ inferred from muon study

TL;DR

This study uses muon spin rotation experiments to explore the microscopic dynamics of hydride ions in LaH$_{3-2x}$O$_x$, revealing their jump motion and suggesting a viscous fluid behavior with a glass transition, which impacts ionic conductivity.

Contribution

It provides new insights into hydride ion dynamics in LaH$_{3-2x}$O$_x$ using $bc$SR, proposing a viscous fluid model over traditional Arrhenius analysis.

Findings

Muon behavior indicates restricted jump motion of H$^-$ ions.

Activation energy for jump motion is 0.11 eV, much lower than conductivity-based estimates.

H$^-$ ions exhibit glass transition-like behavior, affecting ionic conductivity.

Abstract

We performed muon spin rotation and relaxation ($\mu$SR) experiments to investigate the microscopic mechanism behind the high ionic conductivity ($\sigma$) exhibited by hydride (H$^-$) ions in lanthanum hydroxide LaH$_{3-2x}$O$_x$. The $\mu$SR spectra observed at 5--300 K in a sample with $x\approx0.25$ 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 (Mu$_{\rm O}$) appear nearly stationary in the time scale of $\mu$SR ($\sim$10$^{-5}$ s), whereas those at the Tet sites (Mu$_{\rm T}$) 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 Mu$_{\rm T}$ suggest that the jump motion of both Mu$_{\rm T}$ (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 $\sim$1.3 eV evaluated from the temperature dependence of $\sigma$ at high temperatures ($\gtrsim400$ 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 H$^-$ ions can be better explained as a viscous fluid exhibiting a glass transition.