Combining these observations with the changes in the 𝑃 𝜆 , we conclude that He reduces the phonon lifetimes by virtue of a relatively strong anharmonic interaction between the filler and framework modes and an increase in the number of collision channels, while the heavier fillers modulate the number of collision and, to a lesser extent, decay channels. These together produce the intricate variation in the lifetimes seen in the CRTA analysis.