Quasielastic neutron scattering study of tetrahydroborate anion dynamical perturbations in sodium borohydride due to partial halide anion substitution

Equimolar NaBH4-NaX (X = Cl and I) solid solutions were synthesized to study, via quasielastic neutron scattering, the effect of partial halide anion substitution on the reorientational dynamics of tetrahydroborate (BH4) anions in NaBH4. The BH4 reorientational mobility increased in the order of NaBH4-NaCl, NaBH4, and NaBH4-Nal, which corresponded with expanding face-centered-cubic lattices accommodating the respective increasing sizes of the Cl, BH4, and l anions. The BH4 anions in NaBH4-NaCl were found (at least above 400 K) to undergo 'cubic' tumbling motions with the four H atoms per anion visiting all eight corners of a cube, similar to what was previously observed for NaBH4. In contrast, the BH4 anions in NaBH4-Nal were found to undergo something more akin to 'tetrahedral' tumbling motions, where the H atoms visit all four corners of a tetrahedron. Despite a noticeable softening of the BH4 torsional energies with increasing lattice constant amongst NaBH4 and the two solid solutions, all three compounds exhibited similar activation energies for reorientation of about 11-12 kJ mol(-1). Published by Elsevier B.V.