Nuclear Magnetic Resonance Studies of Reorientational Motion and Li Diffusion in LiBH₄–LiI Solid Solutions

Abstract

To study the reorientational motion of the BH4 groups and the translational diffusion of Li+ ions in LiBH4–LiI solid solutions with 2:1, 1:1, and 1:2 molar ratios, we have measured the 1H, 11B, and 7Li NMR spectra and spin–lattice relaxation rates in these compounds over the temperature range 18–520 K. It is found that, at low temperatures, the reorientational motion of the BH4 groups in LiBH4–LiI solid solutions is considerably faster than in all other borohydride-based systems studied so far. Our results are consistent with a coexistence of at least two types of reorientational processes with different characteristic rates. For the faster reorientational process, the average activation energies derived from our data are 53 ± 4, 39 ± 4, and 33 ± 4 meV for the LiBH4–LiI solid solutions with 2:1, 1:1, and 1:2 molar ratios, respectively. In the studied range of iodine concentrations, the Li+ jump rates are found to decrease with increasing I– content. The activation energies for Li diffusion obtained from our data are 0.63 ± 0.01, 0.65 ± 0.01, and 0.68 ± 0.01 eV for the samples with 2:1, 1:1, and 1:2 molar ratios, respectively.