Hydrogen dynamics in β-Mg(BH4)2 on the picosecond timescale

Area of Science:

• Materials Science
• Solid State Physics
• Spectroscopy

Background:

• Magnesium borohydride (Mg(BH4)2) is a promising material for hydrogen storage.
• Understanding hydrogen dynamics is crucial for optimizing its properties.
• Previous studies noted anomalous thermal expansion in β-Mg(BH4)2.

Purpose of the Study:

• To investigate the hydrogen dynamics in β-Mg(BH4)2 on the picosecond timescale.
• To elucidate the contributions of vibrational and rotational motions to hydrogen dynamics.
• To explore the relationship between observed dynamics and thermal expansion.

Main Methods:

• Quasielastic neutron scattering (QENS) was employed.
• Analysis of elastic and quasielastic incoherent structure factors (EISF and QISF).
• Temperature-dependent measurements were performed.

Main Results:

• Both vibrational and rotational motions of [BH4](-) tetrahedra were identified at low energy transfers.
• Below 200 K, these motions were distinct.
• Above 200 K, a dynamic transition was observed, with spectral weight shifting to the quasielastic region.
• No structural phase transition accompanied this dynamic transition.

Conclusions:

• A dynamic transition in hydrogen motion occurs in β-Mg(BH4)2 above 200 K.
• This transition is linked to the anomalous thermal expansion previously reported.
• The findings provide insights into the complex dynamics of Mg(BH4)2 relevant to hydrogen storage applications.