  Rattling dynamics have been identified as fundamental to superconductivity in
defect pyrochlore osmates and aluminium vanadium intermetallics, as well as low
thermal conductivity in clathrates and filled skutterudites. Combining
inelastic neutron scattering (INS) measurements and ab initio molecular
dynamics (MD) simulations, we use a new approach to investigate rattling in the
Al-doped defect pyrochlore tungstates: AAl0.33W1.67O6 (A = K, Rb, Cs). We find
that although all the alkali metals rattle, the rattling of the K atoms is
unique, involving local diffusion within a cage as well as vibrations at a
range of frequencies. We show that the novel K rattling is driven by the strong
anharmonicity and anisotropy of the local potentials around the K atoms, both
of which are a consequence of the small size of the K rattler. Our findings may
open new possibilities for phonon engineering in thermoelectric materials.
