Speakers
Description
Quasielastic neutron scattering was used to investigate the low energy transfer dynamics of the complex borohydrides Mg(BH$_4$)$_2$ in the $\alpha$- and $\beta$-modifications, LiBH$_4$ in the low and high temperature crystal structure, and the 1:1 molar mixture of LiBH$_4$+$\alpha$-Mg(BH$_4$)$_2$. All investigated compounds show a rich dynamic behavior below an energy range of $\Delta E = 10$ meV with the superposition of rotational dynamics of the constituent [BH$_4$]$^-$ anions and low lying lattice modes. For Mg(BH$_4$)$_2$, the rotational diffusion of the [BH$_4$] units was found to be much more activated in the metastable $\beta$-polymorph compared to the $\alpha$-phase, and the low lying lattice modes are even softer in the former crystal structure. In Mg(BH$_4$)$_2$, the structural phase transition is mainly governed by the lattice dynamics, while alkaline LiBH$_4$ exhibits a transition of the [BH$_4$] rotations around the phase transition temperature. Ball milled LiBH$_4$+$\alpha$-Mg(BH$_4$)$_2$ remains a physical mixture of the parent compounds and each component retains its characteristic dynamic signature up to the melting temperature.
