Concomitant Enhancement of the Reorientational Dynamics of the BH ₄ ^– Anions and Mg ²⁺ Ionic Conductivity in Mg(BH ₄ ) ₂ ·NH ₃ 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.