THz range of the phonon spectrum. The 𝑣 𝜆 and 𝑣 𝜆 2 span 3 and 6 orders of magnitude, respectively, and the group velocities reach up to ~5,000 ms -1 . At 300 K the 𝜏 𝜆 range from ~200 ps at low frequencies to < 10 ps for the high-frequency Co-based optic modes. The large 𝜅 𝜆 of the acoustic modes thus arises from their high 𝑣 𝜆 and long 𝜏 𝜆 , while the negligible contribution of the high-frequency optic modes is due to their low 𝑣 𝜆 and a short 𝜏 𝜆 . The Λ 𝜆 approach microns for the acoustic modes but reduce steadily with frequency so that the MFPs of the high-frequency optic modes are on the order of nm. The lifetimes are substantially lowered at 600 K with a corresponding reduction in the MFPs (see Figure .1). It is also of interest to analyse how the 𝜅 λ accumulate as a function of the phonon MFP (Fig. ). At 300 K, around 10 % of the 𝜅 latt is through modes with 𝛬 𝜆 < 10 nm, a further 30 % is through modes with Λ 𝜆 < 100 nm, and 60 % is through modes with MFPs up to 1 𝜇m. At 600 K the MFPs are suppressed due to the reduced lifetimes and modes with Λ 𝜆 from 10-100 nm account for ~70 % of the 𝜅 latt . The effect of limiting the transport through modes with long MFPs can be investigated by using a boundary-scattering model to mimic a finite grain size (Fig. ), and this analysis indicates that nanostructuring can potentially yield a substantial reduction in the 𝜅 latt , in keeping with its widespread use as a strategy for optimising thermoelectric performance. This analysis also highlights the sensitivity of experimental measurements to sample preparation methods. While our calculated bulk 𝜅 latt agree very well with experimental measurements on single crystals, they are 25-30 % higher than measurements on annealed powders. . According to the calculations in Fig. , this reduction could be obtained with a grain size of ~200 nm, which is not unreasonable given that the samples in Ref. were calculated to be 89 % of the theoretical density with an Sb/Co ratio of 2.65.