1 H, 7 Li, and 11 B NMR measurements were used to understand atomic translational motions in both the lowand high-temperature phases (LT, HT) of LiBH 4 . In the HT phase 7 Li spectra, spin-echo T 2 , and T 1 all indicate very rapid lithium ion diffusion. Just above the phase transition, the hydrogen resonance is broad, about 22 kHz fwhm (full width at half of maximum), showing that H translations remain slow. From 120 to 170°C, a rapidly decreasing T 1D (relaxation time of dipolar spin-order) shows that the hydrogens diffuse increasingly rapidly. This motion eventually results in marked hydrogen line-narrowing centered near 190°C; the hydrogen diffusion is likely relevant to the kinetics of dehydriding. The extent of 11 B line-narrowing demonstrates that the boron atoms also diffuse rapidly at temperatures above 200°C. In the LT phase, the hydrogen T 1D decreases rapidly with increasing temperature, here due to 7 Li diffusion which is too slow for line-narrowing.

Hydrogen NMR of LiBH4 in the pores of carbon aerogel nanoscaffolds shows the coexistence of motionally narrowed and broad components. The fraction of mobile, diffusing hydrogen, already evident at room temperature, increases continuously with temperature. Thus, a broad distribution of environments is present, as in some ball-milled hydrides. With decreasing pore size from 25 to 13 nm, the narrowed fraction increases, suggesting that the narrow resonance is from the most defective regions, the grain boundaries. The broad component eventually exhibits narrowing in the same temperature window as for bulk material, confirming the bulk-like structure of those regions. Hole-burning measurements reveal magnetization exchange between the broad and narrow resonance lines, confirming the close spatial proximity of the atoms in each line. The solid−solid transition is clearly evident in 7Li line shapes, with a 10−15 °C depression from the bulk. More rapid decay of the quadrupolar satellite signals in spin echoes, compared to the central transition, is due to lithium atoms diffusing between differently oriented nanocrystallites. Our results suggest that crystallites in neighboring pores have similar orientations but are incoherent for diffraction. Remarkably, the T 1 data of hydrogen and 7Li are continuous in the vicinity of the transition, in contrast with the bulk T 1 data, suggesting that some rapid lithium motion remains below the transition.

In coarse-grained MgH 2 , the diffusive motion of hydrogen remains too slow (<10 5 hops s -1 ) to narrow the H NMR line up to 400 °C. Slow-motion dipolar relaxation time T 1D measurements reveal the motion, with hopping rate ω H from 0.1 to 430 s -1 over the range of 260 to 400 °C, the first direct measurement of H hopping in MgH 2 . The ω H data are described by an activation energy of 1.72 eV (166 kJ/mol) and attempt frequency of 2.5 × 10 15 s -1 . In ball-milled MgH 2 with 0.5 mol % added Nb 2 O 5 catalyst, line-narrowing is evident already at 50 °C. The line shape shows distinct broad and narrow components corresponding to immobile and mobile H, respectively. The fraction of mobile H grows continuously with temperature, reaching ∼30% at 400 °C. This demonstrates that this material's superior reaction kinetics are due to an increased rate of H motion, in addition to the shorter diffusion paths from ball-milling. In ball-milled MgH 2 without additives, the line-narrowed component is weaker and is due, at least in part, to trapped H 2 gas. The spin-lattice relaxation rates T 1 -1 of all materials are compared, with ball-milling markedly increasing T 1 -1 . The weak temperature dependence of T 1 -1 suggests a mechanism with paramagnetic relaxation centers arising from the mechanical milling.