Hydrogen motion inC15-typeZrMo2Hx:Quasielastic neutron scattering and nuclear-magnetic-resonance studies

In order to determine the parameters of H motion in the cubic $(C15\mathrm{}$-type) Laves phase ${\mathrm{ZrMo}}_2,$ we have performed quasielastic neutron scattering measurements and nuclear-magnetic-resonance measurements of the proton spin-lattice relaxation rate on ${\mathrm{ZrMo}}_2{\mathrm{H}}_x$ $(0.3\mathrm{}<~x<~1.0)$ in the temperature range 10–430 K. The diffusive motion of hydrogen in this system can be described in terms of two jump processes: the fast localized H motion within the hexagons formed by interstitial g $\left({\mathrm{Zr}}_2{\mathrm{Mo}}_2\right)$ sites and the slower hopping from one hexagon to the other. The hopping rate ${\tau }_l^{-1}$ of the faster process is found to be of the order of ${10}^{10}\hspace{0.5em}{\mathrm{s}}^{-1}$ at 300 K. The characteristic frequency ${\tau }_d^{-1}$ of the slower process at the same temperature is about $6\times {10}^7\hspace{0.5em}{\mathrm{s}}^{-1}.$ Comparison of the motional parameters of hydrogen in ${\mathrm{ZrMo}}_2{\mathrm{H}}_x$ with those in the related $C15\mathrm{}$-type systems ${\mathrm{TaV}}_2{\mathrm{H}}_x$ and ${\mathrm{ZrCr}}_2{\mathrm{H}}_x$ shows that the value of ${\tau }_d/{\tau }_l$ for ${\mathrm{ZrMo}}_2{\mathrm{H}}_x$ lies between the corresponding values for ${\mathrm{TaV}}_2{\mathrm{H}}_x$ and ${\mathrm{ZrCr}}_2{\mathrm{H}}_x.$ This is consistent with the changes in the ratio of $g-g$ distances $r_1$ (within the hexagons) and $r_2$ (between different hexagons).