Anion and Cation Dynamics in Mixed-Anion Hydroborate Na3(BH4)(B12H12): 1H, 11B, and 23Na NMR Studies

Sodium borohydride-closo-hydroborate Na3(BH4)(B12H12) exhibits the high room-temperature ionic conductivity and high electrochemical stability. To study the dynamical properties of this mixed-anion compound at the microscopic level, we have measured the 1H, 11B, and 23Na nuclear magnetic resonance spectra and nuclear spin-lattice relaxation rates over the temperature range of 8 – 573 K. Our 1H and 11B spin-lattice relaxation measurements have revealed two types of reorientational jump motion. The faster motional process attributed to reorientations of the [BH4]− anions is characterized by the activation energy of 159 meV, and the corresponding reorientational jump rate reaches ~10^8 s^-1 near 130 K. The slower process ascribed to reorientations of the larger [B12H12]− anions is characterized by the activation energy of 319 meV, and the corresponding reorientational jump rate reaches ~10^8 s^-1 near 240 K. The results of the 23Na nuclear magnetic resonance measurements are consistent with the fast long-range diffusion of Na+ ions in Na3(BH4)(B12H12). The diffusive jump rate of Na+ is found to reach ~10^4 s^-1 at 300 K and ~8 × 10^8 s^-1 at 530 K. Comparison of these jump rates with the ionic conductivity data suggests the importance of correlations between diffusing ions.