Effects of partial halide anion substitution on reorientational motion in NaBH 4 : A nuclear magnetic resonance study

Abstract To study the effects of partial halide anion substitution on the reorientational motion of [BH 4 ] − anions in NaBH 4 , we have measured the 1 H and 11 B NMR spectra and spin–lattice relaxation rates in the cubic solid solutions Na(BH 4 ) 0.5 Cl 0.5 and Na(BH 4 ) 0.5 I 0.5 over broad ranges of temperature (8–324 K) and the resonance frequency (14–90 MHz). For both solid solutions, the measured 1 H spin–lattice relaxation rates are governed by reorientations of BH 4 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 1 H spin–lattice relaxation data are 192 ± 7 meV for Na(BH 4 ) 0.5 Cl 0.5 and 120 ± 3 meV for Na(BH 4 ) 0.5 I 0.5 . At a given temperature, the reorientational jump rates are found to increase in the order of Na(BH 4 ) 0.5 Cl 0.5 –NaBH 4 –Na(BH 4 ) 0.5 I 0.5 , 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 − –[BH 4 ] − –I − ).