Evidence for independent motional processes on the two interstitial sublattices of a layer-structured metal hydride: Hydrogen spin-lattice relaxation and motional narrowing in zirconium monohalide hemihydrides,ZrXH0.5

We report the results of a proton-magnetic-resonance investigation of hydrogen location and motion in the hemihydrides $\mathrm{Zr}X{\mathrm{H}}_{0.5}$ of the metallic layer-structured monohalides $\mathrm{Zr}X$ of zirconium ($X=\mathrm{B}\mathrm{r},\mathrm{C}\mathrm{l}$). Wide-line and pulsed NMR methods were employed to measure the temperature dependence of the linewidth and second moment and of the spin-lattice relaxation time in the laboratory and rotating frames. The results indicate that hydrogen forms an ordered structure on the tetrahedral ($T$) interstitial sublattice within the Zr metal bilayers, with some (small) random occupancy of octahedral ($O$) sites. Two stages of motional narrowing observed in the wide-line measurements and double minima found in the relaxation times are consistent with the occurrence of essentially independent hydrogen motional processes on the $T$ and $O$ interstitial sublattices. Hydrogen site occupancy probabilities, jump frequencies, activation energies for hydrogen diffusion, and conduction-electron contributions to the proton spin-lattice relaxation rate are deduced from the measurements.