Effects of partial halide anion substitution on reorientational motion in NaBH4: A nuclear magnetic resonance study

To study the effects of partial halide anion substitution on the reorientational motion of [BH4]− anions in NaBH4, we have measured the 1H and 11B NMR spectra and spin-lattice relaxation rates in the cubic solid solutions Na(BH4)0.5Cl0.5 and Na(BH4)0.5I0.5 over broad ranges of temperature (8-324 K) and the resonance frequency (14-90 MHz). For both solid solutions, the measured 1H spin-lattice relaxation rates are governed by reorientations of BH4 groups, and the experimental data can be satisfactorily described by the model with a Gaussian distribution of the activation energies. The average values of the activation energies derived from the 1H spin-lattice relaxation data are 192 ± 7 meV for Na(BH4)0.5Cl0.5 and 120 ± 3 meV for Na(BH4)0.5I0.5. At a given temperature, the reorientational jump rates are found to increase in the order of Na(BH4)0.5Cl0.5-NaBH4-Na(BH4)0.5I0.5, in agreement with the quasielastic neutron scattering (QENS) results. This trend correlates with the lattice expansion reflecting the increase in the size of the corresponding anions (in the order of Cl−-[BH4]−-I−).