Nuclear magnetic resonance study of atomic motion in the mixed borohydride-amide Li-2(BH4) (NH2)

摘要

In order to study the reorientational motion of the anions and the translational diffusion of Li+ cations in the borohydride-amide Li-2(BH4) (NH2) showing fast-ion conduction, we have measured the H-1, Li-7, and B-11 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 H-1, Li-7, and B-11 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 similar to 10(8) 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 Li-7 NMR line narrowing above 300 K that indicates the onset of Li+ jumps at the frequency scale of similar to 10(4) s(-1). As the temperature approaches the melting point of Li-2(BH4) (NH2), T-m approximate to 365 K, we have also observed the onset of diffusive motion of H-containing species at the same frequency scale. (C) 2020 Elsevier B.V. All rights reserved.