NMR Study of the Dynamical Properties of LiLa(BH4)3Br and LiLa(BH4)3I

Metal borohydrides have attracted significant attention due to their potential hydrogen storage properties and their prospective applications as solid-state electrolytes in batteries. Dynamical properties of these materials are of great importance for high ionic conductivity, which is the focus of the present investigation. To study the reorientational motion of BH4 groups and the translational diffusion of Li+ ions in the novel bimetallic borohydrides–halides LiLa(BH4)3X (X = Br, I), we have measured the 1H and 7Li NMR spectra and spin–lattice relaxation rates in these compounds over the temperature range of 5–418 K. At low temperatures (T < 110 K), the fast reorientations of BH4 groups are observed, and the corresponding jump rate is found to increase with increasing halide ion radius. For this reorientational jump process, the characteristic activation energies derived from our data are 28(5) meV for LiLa(BH4)3Br and 21(5) meV for LiLa(BH4)3I. Above 200 K, the translational diffusion of Li ions is observed at the NMR frequency scale, which is consistent with high ionic conductivity of these compounds. The activation energies for Li diffusion obtained from our data are 285(5) meV for LiLa(BH4)3Br and 262(4) meV for LiLa(BH4)3I.