Reorientational Motion of BH4 Ions in Alkali Borohydrides MBH4 (M = Li, Na, K) as Studied by Solid-State NMR

Ion dynamics in alkali borohydrides, LiBH4, NaBH4, and KBH4, was studied by solid-state NMR. NMR spectra and spin-lattice relaxation times, T-1, of H-1 and B-11 were measured and analyzed. The T-1 results of H-1 and B-11 indicated that two types of motions take place in the low-temperature (LT) phase of LiBH4. The second moments of H-1 and B-11 NMR spectra suggested that the sum of the two types of motions resembles isotropic reorientation of the BH4 unit. The T-1 minimum values of H-1 and B-11 supported that the C-3 rotation takes place at first. The second type of motion might be either C-3 rotation or C-2 rotation. The second moments of NMR spectra and the T-1 minimum values of H-1 and B-11 supported that the BH4 unit reorients isotropically in NaBH4, and KBH4.