Nuclear magnetic resonance study of atomic motion in the mixed borohydride-amide Li2(BH4) (NH2)

Abstract

In order to study the reorientational motion of the anions and the translational diffusion of Li+ cations in the borohydride-amide Li2(BH4) (NH2) showing fast-ion conduction, we have measured the 1H, 7Li, and 11B NMR spectra and spin-lattice relaxation rates over the temperature range of 80–341 K. Our measurements have revealed a coexistence of several motional processes in this mixed-anion compound. The fastest processes correspond to two thermally-activated types of BH4 reorientations. These reorientations give rise to two peaks of the 1H, 7Li, and 11B spin-lattice relaxation rates observed near 220 K and 290 K; they are characterized by the activation energies of 196 (2) meV and 498 (5) meV, respectively. For each of the two processes, the reorientational jump rate reaches ∼108 s−1 near the corresponding peak temperature. The diffusive jump motion of Li+ ions is found to be much slower than both reorientational processes; it leads to the 7Li NMR line narrowing above 300 K that indicates the onset of Li+ jumps at the frequency scale of ∼104 s−1. As the temperature approaches the melting point of Li2(BH4) (NH2), Tm ≈ 365 K, we have also observed the onset of diffusive motion of H-containing species at the same frequency scale.