Nuclear magnetic resonance studies of atomic motion in borohydrides

Two basic types of thermally activated atomic jump motion are known to exist in solid borohydrides: the fast reorientations of BH4 groups and the slower translational diffusion of cations and anions. This paper reviews recent progress in nuclear magnetic resonance (NMR) studies of both these jump processes. It is shown that, for some borohydrides, NMR measurements of the nuclear spin-lattice relaxation can trace the changes in the reorientational jump rates over the range of eight orders of magnitude (104 to 1012 s−1). Such a wide dynamic range leads to the high precision of the activation energies for BH4 reorientations, as derived from the nuclear spin-lattice relaxation measurements.

Research highlights▶ RbBH4 and KBH4: a single jump process of BH4 reorientations. ▶ NaBH4: the effect of the structural phase transition on BH4 reorientations. ▶ Low-T phase of LiBH4: coexistence of two jump processes of BH4 reorientations. ▶ α-Mg(BH4)2: complex reorientational behavior due to the unusual coordination of BH4. ▶ High-T phase of LiBH4: translational diffusion of Li and BH4 units.