Model-Independent Measurements of Hydrogen Diffusivity in the Lanthanum Dihydride-Trihydride System

First model-independent measurements of the hydrogen diffusivity $D$ in lanthanum trihydrides ${\mathrm{LaH}}_{x}$ $(2.00\ensuremath{\le}x\ensuremath{\le}3.0)$ have been performed by PFG-NMR. At a fixed concentration $x\ensuremath{\le}2.92$, $D(T)$ shows single Arrhenius behavior over wide temperature ranges. The activation enthalpy decreases with increasing $x$ from 0.55 $(x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.00)$ to 0.17 eV $(x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.92)$, resulting in a strong increase in $D$. The formation of a hydrogen superstructure associated with the semiconductor-to-metal transition results in a strong reduction in $D$. It was also observed for the first time that in the stoichiometric limit $D$ depends on the thermal history of the sample.