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

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), T_m ≈ 365 K, we have also observed the onset of diffusive motion of H-containing species at the same frequency scale.