Alternatively, fillers of a similar chemical nature to the heavier noble gases could be combined with a second filler to suppress both acoustic-and optic-mode transport -this strategy of introducing two fillers with different rattling frequencies has proven successful in experiments. Given that ~80 % of the heat transport in CoSb3 is through the acoustic modes, the resonant-scattering and avoided-crossing mechanisms both predict that lowering the rattling frequency below 2.5 THz would produce a larger reduction in 𝜅 latt . This is not possible with the five fillers studied here because the heavier noble gases interact more strongly with the framework. We can, however, perform a "thought experiment" to decouple the mass and interaction strength by changing the mass of the fillers while keeping the second-and third-order force constants fixed, and vice versa.