  Skutterudites are crystals with a cage-like structure that can be augmented
with filler atoms ("rattlers"), usually leading to a reduction in thermal
conductivity that can be exploited for thermoelectric applications. Here, we
leverage the recently introduced Wigner formulation of thermal transport to
elucidate the microscopic physics underlying heat conduction in skutterudites,
showing that filler atoms can drive a crossover from the Boltzmann to the
Wigner regimes of thermal transport, i.e., from particle-like conduction to
wave-like tunnelling. At temperatures where the thermoelectric efficiency of
skutterudites is largest, wave-like tunneling can become comparable to
particle-like propagation. We define a Boltzmann deviation descriptor able to
differentiate the two regimes and relate the competition between the two
mechanisms to the materials' chemistry, providing a design strategy to select
rattlers and identify optimal compositions for thermoelectric applications.
