Abstract
Polyhydroborate salts represent the important class of energy materials attracting significant recent attention. Some of these salts exhibit promising hydrogen storage properties and/or high ionic conductivities favorable for applications as solid electrolytes in batteries. Two basic types of thermally activated atomic jump motion are known to exist in these materials: the reorientational (rotational) motion of complex anions and the translational diffusion of cations or complex anions. The present paper reviews recent progress in nuclear magnetic resonance (NMR) studies of both reorientational and diffusive jump motion in polyhydroborate salts. The emphasis is put on sodium and lithium closo-borates exhibiting high ionic conductivity and on borohydride-based systems showing extremely fast reorientational motion down to low temperatures. For these systems, we discuss the effects of order–disorder phase transitions on the parameters of reorientations and diffusive jumps, as well as the mechanism of low-temperature rotational tunneling.
Highlights
Polyhydroborate salts are the ionic compounds described by a general formula Mx [Bm Hn ]y, where M is a metal cation and [Bm Hn ] is a complex anion composed of boron and hydrogen atoms, such as [BH4 ]−, [B10 H10 ]2−, or [B12 H12 ]2−
The emphasis is put on sodium and lithium closo-hydroborates and the related systems exhibiting high ionic conductivity and on borohydride-based systems showing extremely fast reorientational motion down to low temperatures
This brief review emphasizes the potential of nuclear magnetic resonance (NMR) for the investigation of a rich picture of anion anion and cation dynamics in polyhydroborate salts
Summary
Polyhydroborate salts are the ionic compounds described by a general formula Mx [Bm Hn ]y , where M is a metal cation and [Bm Hn ] is a complex anion composed of boron and hydrogen atoms, such as [BH4 ]− , [B10 H10 ]2− , or [B12 H12 ]2−. Light-metal tetrahydroborates based on the [BH4 ]− anion are considered as promising materials for hydrogen storage due to their high volumetric and gravimetric hydrogen densities [3]; these compounds are often referred to as “borohydrides” Another area of potential applications of hydroborate salts is related to their electrical conduction properties. The emphasis is put on sodium and lithium closo-hydroborates and the related systems exhibiting high ionic conductivity and on borohydride-based systems showing extremely fast reorientational motion down to low temperatures For these systems, we discuss the relation between the anion reorientations and the cation diffusion, the effects of the order–disorder phase transitions on the motional parameters, as well as the mechanism of low-temperature rotational tunneling
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