RESUMO
Quasielastic neutron scattering was used to investigate the low energy transfer dynamics of the complex borohydrides Mg(BH4)2 in the α- and ß-modifications, LiBH4 in the low and high temperature crystal structure, and an 1 : 1 molar mixture of LiBH4 + α-Mg(BH4)2. All investigated compounds show a rich dynamic behaviour below an energy range of ΔE = 10 meV with the superposition of rotational dynamics of the constituent [BH4]- anions and low lying lattice modes. For Mg(BH4)2, the rotational diffusion of the [BH4] units was found to be much more activated in the metastable ß-polymorph compared to the α-phase, and the low lying lattice modes are even softer in the former crystal structure. In Mg(BH4)2, the structural phase transition is mainly governed by the lattice dynamics, while alkaline LiBH4 exhibits a transition of the [BH4] rotations around the phase transition temperature. Ball milled LiBH4 + α-Mg(BH4)2 remains a physical mixture of the parent compounds and each component retains its characteristic dynamic signature up to the melting temperature.
RESUMO
A quasielastic neutron scattering study on ß-Mg(BH4)2 has been performed to investigate the hydrogen dynamics on the picosecond time-scale. Both vibrational and rotational motions of the [BH4](-) tetrahedra contribute to the signal at low energy transfers. A comprehensive analysis of the elastic and quasielastic incoherent structure factors allowed the separation of different parts. Below 200 K, vibrations and rotations (around the C2 or C3 symmetry axis of the [BH4](-) tetrahedra) are well separated. Above that temperature, a transition is observed in the vibrational part, and the spectral weight is shifted towards the quasielastic region. The dynamic transition is not accompanied by any structural phase change but we suggest that it is correlated with the anomalous thermal expansion that has been reported for ß-Mg(BH4)2 [Filinchuk, et al., Chem. Mater., 2009, 21, 925].