Dynamic enhancement of cation migration in a Zintl alloy by polyanion rotation

ZINTL alloys are compounds of an alkali metal A with a polyvalent metal M in which transfer of an electron from A to M leads to bonding behaviour in the M^- ions typical of elements one column to the right in the Periodic Table. For example, ASn and APb contain tetrahedral M^4-₄ polyanions in both the crystalline and liquid phases¹ whereas AA1 alloys crystallize in tetrahedrally bonded networks typical of Group IV elements². These bonding patterns also determine the character of the disorder in the solid at high temperature. We have recently identified a plastic crystal phase in CsPb just below the melting point, in which the Pb ⁴₄ polyanions undergo jump reorientations³; LiAl, in contrast, is metallic, with mobile Li⁺ ions⁴. Here we report unusual disordering behaviour in the Zintl alloy NaSn, in which both types of dynamic disorder appear: rapid reorientations of the Sn ^4-₄ polyanions enhance jump migration of the Na ⁺ cations by means of a 'paddle-wheel' effect. The two types of disorder are strongly coupled, so that only one disordering transition takes place as the melting point is approached.