We also note that lowering the masses of Kr and Xe, with the strongest filler-framework interactions, to that of He produces an unphysically large 𝜅 latt . This combination results in rattling frequencies of 17.4 and 20.7 THz, which are both so far outside the frequency range of the CoSb3 spectrum that there would be very few energy-conserving scattering channels to limit their modal 𝜅 𝜆 .