Concomitant Enhancement of the Reorientational Dynamics of the BH4 – Anions and Mg2+ Ionic Conductivity in Mg(BH4)2·NH3 upon Ligand Incorporation

TL;DR

Adding ammonia to a magnesium compound increases ion conductivity and the movement of certain anions, which may be linked.

Contribution

The study reveals a link between enhanced Mg2+ conductivity and BH4– anion reorientational dynamics upon NH3 addition.

Findings

NH3 ligand increases Mg2+ ionic conductivity and BH4– anion reorientational mobility.

Terminal BH4– anions have a much lower reorientational energy barrier than bridging anions.

NH3 ligands exhibit faster reorientational dynamics and quantum tunneling below 50 K.

Abstract

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.

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