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

Sodium borohydride-closo-hydroborate Na-3(BH4)(B12H12) exhibits 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 H-1, B-11, and 23Na nuclear magnetic resonance spectra and nuclear spin-lattice relaxation rates over the temperature range of 8-573 K. Our H-1 and B-11 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 an activation energy of 159 meV, and the corresponding reorientational jump rate reaches similar to 10(8) s(-1) near 130 K. The slower process ascribed to reorientations of the larger [B12H12](-) anions is characterized by an activation energy of 319 meV, and the corresponding reorientational jump rate reaches similar to 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 Na-3(BH4)(B12H12). The diffusive jump rate of Na+ is found to reach similar to 10(4) s(-1) at 300 K and similar to 8 x 10(8) s(-1) at 530 K. A comparison of these jump rates with the ionic conductivity data suggests the importance of correlations between diffusing ions.