Nuclear Magnetic Resonance Studies of BH₄ Reorientations and Li Diffusion in LiLa(BH₄)₃Cl

To study the reorientational motion of BH₄ groups and the translational diffusion of Li⁺ ions in the novel bimetallic borohydride chloride LiLa­(BH₄)₃Cl, we have measured the ¹H, ¹¹B, and ⁷Li NMR spectra and spin–lattice relaxation rates in this compound over the temperature range of 23–418 K. At low temperatures (T < 110 K), the proton spin–lattice relaxation rates are governed by fast reorientations of BH₄ groups. This reorientational process can be satisfactorily described in terms of a two-peak distribution of the activation energies with the peak E_a values of 41 and 50 meV. Above 200 K, the NMR data are governed by a combined effect of two types of motion occurring at the same frequency scale: Li ion diffusion and another (slower) reorientational motion of BH₄ groups. These results suggest that the Li ion jumps and the slower reorientational jumps of BH₄ groups in LiLa­(BH₄)₃Cl may be correlated. The estimate of the tracer Li ion diffusion coefficient at room temperature (5.2 × 10^–8 cm²/s) following from our experimental data indicates that LiLa­(BH₄)₃Cl can be considered as a promising solid-state ionic conductor.