Reorientational Dynamics of NaBH4 and KBH4

The details of the rotational dynamics of borohydride ions in both the ordered and disordered crystal phases of NaBH4 and in the disordered crystal phase of KBH4 were determined by quasielastic neutron scattering (QNS). Model fits of the QNS data indicate that the BH4- tetrahedron in the ordered phase of NaBH4 rotates with a combination of 2-site and 3-site reorientations that preserve its crystallographic orientation. The QNS results for the disordered phases are well described by a model assuming nearest-neighbor BH4- jumps from one corner to another of a cube formed by eight hydrogen positions of half occupancy. Distinguishing between likely mechanisms for reorientation was made possible by collecting data at sufficiently high momentum transfer. The activation energies derived for the low-temperature and high-temperature phases of NaBH4 are 13.4 +/- 0.8 and 11.9 +/- 0.5 kJ/mol, respectively. We find an activation energy for KBH4 of 14.6 +/- 0.5 kJ/mol in the high-temperature phase. The torsional vibration bands of both borohydrides were also measured by inelastic neutron scattering.