Mapping trends in the reorientational mobilities of tetrahydroborate anions via neutron-scattering fixed-window scans

► Reorientations of BH4− anions were probed via neutron fixed-window scans (FWSs). ► Both bulk and destabilized alkali-metal borohydride MBH4 compounds were measured. ► The T profile of the FWS was a good indicator of BH4− reorientational mobilities. ► BH4− mobility in MBH4 increased considerably with cation size from Li+ to Cs+. ► Nanoconfined and LiI-alloyed LiBH4 showed enhanced BH4− mobilities versus bulk LiBH4.