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

Abstract Equimolar NaBH 4 –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 (BH 4 − ) anions in NaBH 4 . The BH 4 − reorientational mobility increased in the order of NaBH 4 –NaCl, NaBH 4 , and NaBH 4 –NaI, which corresponded with expanding face-centered-cubic lattices accommodating the respective increasing sizes of the Cl − , BH 4 − , and I − anions. The BH 4 − anions in NaBH 4 –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 NaBH 4 . In contrast, the BH 4 − anions in NaBH 4 –NaI 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 BH 4 − torsional energies with increasing lattice constant amongst NaBH 4 and the two solid solutions, all three compounds exhibited similar activation energies for reorientation of about 11–12 kJ mol −1 .