The Influence\nof Reorientational and Vibrational Dynamics\non the Mg<sup>2+</sup> Conductivity in Mg(BH<sub>4</sub>)<sub>2</sub>·CH<sub>3</sub>NH<sub>2</sub>

Reorientational dynamics in solid electrolytes can significantly\nenhance the ionic conductivity, and understanding these dynamics can\nfacilitate the rational design of improved solid electrolytes. Additionally,\nrecent investigations on metal hydridoborate-based solid electrolytes\nhave shown that the addition of a neutral ligand can also have a positive\neffect on the ionic conductivity. In this study, we investigate the\ndynamics in monomethylamine magnesium borohydride (Mg(BH<sub>4</sub>)<sub>2</sub>·CH<sub>3</sub>NH<sub>2</sub>) with quasielastic\nand inelastic neutron scattering, density functional theory calculations,\nand molecular dynamics simulations. The results suggest that the addition\nof methylamine significantly speeds up the reorientational frequency\nof the BH<sub>4</sub><sup>–</sup> anion compared to Mg(BH<sub>4</sub>)<sub>2</sub>. This is likely part of the explanation for\nthe high Mg-ion transport observed for Mg(BH<sub>4</sub>)<sub>2</sub>·CH<sub>3</sub>NH<sub>2</sub>. Furthermore, while the dynamics\nof both the BH<sub>4</sub><sup>–</sup> anion and the CH<sub>3</sub> group of the methylamine ligand is rapid, the NH<sub>2</sub> group of the methylamine ligand exhibits much slower reorientations,\nas confirmed by both experimental and computational investigations.\nNotably, molecular dynamics calculations reveal mean square displacements\nof 0.387 Å<sup>2</sup> for NH<sub>2</sub>, 1.503 Å<sup>2</sup> for CH<sub>3</sub>, and 1.856 Å<sup>2</sup> for BH<sub>4</sub><sup>–</sup> using a trajectory of 10 ps. This study confirms\nthe simultaneous presence of fast dynamics and high ionic conductivity\nin a metal borohydride-based system and can function as an experimental\nfoundation for future studies on dynamics in hydrogen-rich solid electrolytes.