A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth borohydrides LiBH4 and Mg(BH4)2 and the mixture LiBH4 + Mg(BH4)2.

Luca Silvi,Wiebke Lohstroh,Maximilian Fichtner,Zhirong Zhao-Karger,Eva Röhm,Winfried Petry

doi:10.1039/c8cp04316g

Abstract

Quasielastic neutron scattering was used to investigate the low energy transfer dynamics of the complex borohydrides Mg(BH4)2 in the α- and β-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 β-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.

Highlights

Complex tetraborohydrides are a class of materials that have been studied intensely for their high volumetric and gravimetric hydrogen content and their potential use in solid state hydrogen storage applications
Complex hydrides are salt like materials, composed of [BH4]À anions where four hydrogen atoms are covalently bound around the central boron atom, and the positively charged alkaline or alkaline-earth cation
Their crystal structures and thermodynamic stability are governed by the structural arrangement of the [BH4] units, yielding to rich phase diagrams especially for the alkaline-earth compounds

Summary

Introduction

Complex tetraborohydrides are a class of materials that have been studied intensely for their high volumetric and gravimetric hydrogen content and their potential use in solid state hydrogen storage applications. The most investigated compounds are lithium-, magnesium-, and calcium borohydrides which exhibit high hydrogen contents of 18.4 wt% H2 (LiBH4), 14.9 wt% H2 (Mg(BH4)2), and 11.6 wt% H2 (Ca(BH4)2), respectively.[1,2,3,4,5] In general, complex hydrides are salt like materials, composed of [BH4]À anions where four hydrogen atoms are covalently bound around the central boron atom, and the positively charged alkaline or alkaline-earth cation Their crystal structures and thermodynamic stability are governed by the structural arrangement of the [BH4] units, yielding to rich phase diagrams especially for the alkaline-earth compounds. The 1 : 1 ratio is close to the eutectic composition of the binary system with a melting point between 450 and 520 K.30

Experimental

Data analysis

Results and discussion

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