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

•NaBH4–NaX (X=Cl, I) solutions were made by ball-milling/annealing pure compounds.•BH4− reorientational motions were studied by quasielastic neutron scattering.•Mobility increased from X=Cl to NaBH4 to X=I, consistent with expanding lattices.•Near 400K, BH4− favored cubic tumbling for X=Cl and tetrahedral tumbling for X=I.•Activation energies were in the range of 11–12kJmol−1 for both compounds.