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