Quasielastic neutron scattering measurements on the Zintl alloy NaSn s
how that the high-temperature solid phase discovered by Hume-Rothery i
s a plastic crystal characterized by rapid rotations of the Sn4{4-} Zi
ntl ions. The reorientations are similar to those previously observed
in CsPb but have a markedly different wave vector dependence, indicati
ng a different behaviour of the alkali metal ions.