Slow H hopping motions in MgH$_{2}$ and alanates

Unlike the interstitial (metallic) metal-hydrides, ionic and/or complex hydrides such as MgH$_{2}$ and NaAlH$_{4}$ have very slow rates of H atomic hopping. Because the rates are too small ($<$10$^{5}$ s$^{-1})$ for motional narrowing of the dipolar-broadened H NMR lines, we report here the rates of motion from T$_{1D}$ slow-motion measurements. The activation energy for H motion in MgH$_{2}$ has been determined to be 1.45 eV. In undoped NaAlH$_{4}$, the rate of motion is also thermally activated. In ScCl$_{3}$-doped NaAlH$_{4}$, much faster motion (shorter T$_{1D})$ is found, even at low-temperature. At low-T, the role of rotating AlH$_{6}$ groups formed by partial dehydriding is suspected.