Abstract
The addition of neutral ligand NH(3) is known to increase the Mg(2+) ionic conductivity in Mg-(BH(4))(2)·NH(3) as compared to the parent compound Mg-(BH(4))(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 Mg(2+) ionic conductivity were investigated. The results show that the introduction of the NH(3) ligand not only enhances the Mg(2+) ionic conductivity but also significantly increases the reorientational mobility of the BH(4) (-) anions. Thus, the results suggest that there may be a link between the two. Furthermore, the results show that Mg-(BH(4))(2)·NH(3) exhibits two coordination environments for the BH(4) (-) anions, which act as either bridging or terminal anions, in contrast to Mg-(BH(4))(2), which only exhibits bridging anions. The different coordination environments in Mg-(BH(4))(2)·NH(3) 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 NH(3) ligands also exhibit reorientational dynamics and that these are even faster than the dynamics of the BH(4) (-) anions, with the NH(3) ligands having a reorientational energy barrier of ∼10 meV. In addition to the reorientational dynamics, the NH(3) 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-(BH(4))(2)·NH(3) and suggests that the rapidly reorienting terminal BH(4) (-) anions may be behind the increased Mg(2+) ionic conductivity upon ligand incorporation.