LiBH4 in Carbon Aerogel Nanoscaffolds: An NMR Study of Atomic Motions

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 rim, 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 Li-7 line shapes, with a 10-15 degrees 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 Li-7 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.