The 16 atoms in the primitive cell produce 3𝑁 = 48 branches at each phonon wavevector. The three acoustic modes span a frequency range of ~2.5 THz. The 48 modes can be partitioned into low-and high-frequency groups in a 3:1 ratio separated by a "phonon bandgap". The lower-frequency group extends up to ~6 THz, while the upper-frequency group spans from ~7-8.5 THz. With reference to the atom-projected phonon DoS, these bands can be assigned as predominantly vibrations involving Sb and Co motion respectively. Our results are a good match to experimental data and to other modelling studies. The lattice thermal conductivity 𝜿 latt as a function of temperature, calculated using the single-mode relaxation-time approximation (RTA) model, is shown in Fig. . The diagonal elements 𝜅 𝑥𝑥 , 𝜅 𝑦𝑦 and 𝜅 𝑧𝑧 of the 3 × 3 𝜿 latt tensor correspond to the transport along the principal Cartesian directions. In a cubic system, the diagonal elements are equal, so for simplicity we discuss the scalar diagonal average 𝜅 latt = 1 3 Tr[𝜿 latt ]. The calculations are an excellent match to measurements on single crystals with modest doping levels, and the predicted 9.98 and 4.91 W m -1 K -1 at 𝑇 = 300 and 600 K, respectively, agree quantitatively with the experimental measurements of 10 W m -1 K -1 and 5-5.5 W m -1 K -1 in Ref. . The calculations and measurements diverge beyond 600 K due to a plateau in the measured 𝜅 latt , which may be due to the neglect of higher-order anharmonic effects and/or thermal expansion in the RTA, or to other thermal effects such as the onset of oxidation or decomposition. Figure (a) Conventional unit cell of CoSb3 comprising a network of corner-sharing CoSb6 octahedra (dark blue/brown) surrounding cavities centred at (0, 0, 0) and (0.5, 0.5, 0.5). (b) Representative XCo8Sb24 filled model with the filler atom X in the central cavity (pale blue). These images were prepared using the VESTA software. Figure Calculated phonon dispersion and density of states 𝑔(𝑓) of CoSb3. The projections of the 𝑔(𝑓) onto the Co and Sb atoms is shown by the blue and red shaded curves respectively. Experimental measurements of the Γ-point phonon frequencies from infrared (IR) and Raman spectroscopy and measurements of the frequencies along the Γ → H segment of the phonon dispersion from inelastic X-ray scattering (IXS) are overlaid for comparison. and are overlaid for comparison.