During post-processing, phonon density of states (DoS) curves 𝑔(𝑓) were computed by interpolating the frequencies onto a uniform Γ-centered grid with 48 × 48 × 48 subdivisions and applying a Gaussian broadening with 𝜎 = 0.032 THz, corresponding to a full-width at half-maximum of 2.5 cm -1 . Atom-projected phonon DoS (PDoS) curves were computed on 24 × 24 × 24 grids. For pristine CoSb3 a transformation matrix was used to project to the primitive cell, and the grids were increased to 60 × 60 × 60 and 30 × 30 × 30, respectively, to account for the 2× larger Brillouin zone. The phonon dispersion of undoped CoSb3 was obtained by evaluating the frequencies along paths connecting the high-symmetry wavevectors in the 𝐼𝑚3 ̅ Brillouin zone.