Atomic Motions in Ionic Hydrides: MgH$_{2}$, NaMg$_{3}$H$_{3}$, and LiBH$_{4}$

In hydrogen storage, rapid hydrogen diffusion is a key component for rapid reaction rates of dehydriding and rehydriding. In metallic systems, the light interstitial H atoms typically do display rapid diffusion. However, recent interest has focused on ionic and complex hydrides of light metal-atoms because of their high weight fractions of reversible hydrogen. These ionic complex hydrides generally reveal slow hydrogen diffusion and resultingly slow reaction kinetics. We report here studies of H diffusion using NMR in several such hydrides. In MgH$_{2}$, the rate $\omega _{H}$ of H hopping remains too slow to narrow the H NMR up to 400 $^{\circ}$C. T$_{1D}$ measurements, however, can detect the motion and find an activation energy of 1.72 eV, the first reported direct measurement of diffusion in MgH$_{2}$. In ball-milled (bm) material with Nb$_{2}$O$_{5}$ catalyst additive, a fraction of the resonance intensity is narrowed starting at 50 $^{\circ}$C, with the narrow fraction growing to 30{\%} by 400 $^{\circ}$C. A model for continuous growth of the narrow line, based on a wide distribution of motion rates, is presented. Ball-milling also greatly increases the laboratory-frame relaxation rates, T$_{1}^{-1}$, from paramagnetic defects created by the mechanical process. In bm NaMgH$_{3}$, an even larger fraction of the resonance is motionally-narrowed, growing to nearly 100{\%} by 300 $^{\circ}$C. Clearly, ball-milling has a much more profound effect on ionic hydrides than the simple reduction of grain sizes and diffusion distances. In coarse-grain LiBH$_{4}$ (with 13.8 weight{\%} reversible hydrogen), an orientationally disordered solid phase occurs above 110 $^{\circ}$C. Above the transition, the rate of Li ion diffusion increases remarkably. H diffusion starts to narrow the H NMR line around 170 $^{\circ}$C, continuing to narrow up to the melt near 280 $^{\circ}$C. To distinguish diffusion of (already rapidly rotating) BH$_{4}$ units from H exchange between neighboring BH$_{4}$, the $^{11}$B resonance was studied. The boron line central transition becomes much narrower (400 Hz) than the width (1500 Hz) expected from Van Vleck M$_{2}$ for the case of static boron spins (with rapid Li and H diffusion). Thus, intact BH$_{4}$ units are the diffusing species. Even in molten LiBH$_{4}$, the BH$_{4}$ lifetime is found to be at least 2 seconds from observations of the B-H J-coupling pattern, so it is probably much larger in the solid.