Concomitant Enhancement of the Reorientational Dynamics of the BH4 - Anions and Mg2+ Ionic Conductivity in Mg(BH4)2·NH3 upon Ligand Incorporation.

The addition of neutral ligand NH3 is known to increase the Mg2+ ionic conductivity in Mg-(BH4)2·NH3 as compared to the parent compound Mg-(BH4)2. Using inelastic neutron scattering, quasielastic neutron scattering, synchrotron X-ray powder diffraction, impedance spectroscopy, and density functional theory, the structure, the dynamics, and the Mg2+ ionic conductivity were investigated. The results show that the introduction of the NH3 ligand not only enhances the Mg2+ ionic conductivity but also significantly increases the reorientational mobility of the BH4 - anions. Thus, the results suggest that there may be a link between the two. Furthermore, the results show that Mg-(BH4)2·NH3 exhibits two coordination environments for the BH4 - anions, which act as either bridging or terminal anions, in contrast to Mg-(BH4)2, which only exhibits bridging anions. The different coordination environments in Mg-(BH4)2·NH3 lead to a clear difference in dynamics where the terminal anions have a much lower reorientational energy barrier (∼65 meV), as compared to the bridging anions (∼280 meV), and thus become dynamically active at much lower temperatures. The results show that the NH3 ligands also exhibit reorientational dynamics and that these are even faster than the dynamics of the BH4 - anions, with the NH3 ligands having a reorientational energy barrier of ∼10 meV. In addition to the reorientational dynamics, the NH3 ligands undergo quantum mechanical rotational tunneling below 50 K. In summary, this study provides a detailed characterization of both the structure and the dynamics of Mg-(BH4)2·NH3 and suggests that the rapidly reorienting terminal BH4 - anions may be behind the increased Mg2+ ionic conductivity upon ligand incorporation.