Accumulating the 𝜅 λ as a function of frequency and comparing the result to the phonon DoS shows how modes in different frequency ranges contribute to the overall 𝜅 latt (Fig. ). At 300 K, modes up to 2.5 THz contribute ~ 80 % to the 𝜅 latt , and the remaining 20 % is accumulated up to ~6 THz. The majority of the thermal transport is therefore through the acoustic modes and the remainder is through the lower-frequency Sb-based optic modes (c.f. Fig. ). The contribution of the high-frequency Co-based modes is negligible. Analysis of the 𝜅 latt at 600 K yields similar conclusions but with a marked reduction in the 𝜅 λ across the frequency spectrum. Further insight into the nature of the 𝜅 latt can be obtained from the frequency spectra of the modal terms in Eq. 2 and the mean-free paths 𝚲 𝜆 (Fig. ). We consider the group velocity/mean free path norms