As for 𝜅 𝜆 , the 𝜅 𝜆 𝜏 𝜆 ⁄ in the summand in Eq. 6 can be accumulated as a function of frequency (Fig. , Fig. .12). This shows that the largest change in the 𝜅 latt 𝜏 CRTA ⁄ on filling occurs in the optic modes between ~2.5-6 THz. Comparing accumulations of 𝜅 𝜆 𝜏 𝜆 ⁄ and 𝜅 𝜆 , as in Fig. , shows a small reduction in heat transport through the acoustic modes between 1-2.5 THz and a larger reduction in transport through the optic modes, which is also reflected in the 𝜅 latt 𝜏 CRTA ⁄ . Taking these analyses together, we conclude that the five fillers primarily suppress the heat transport in CoSb3 by reducing the group velocity and lifetimes of the optic modes.