CoSb3 is cubic with the 𝐼𝑚3 ̅ spacegroup. The 32-atom conventional unit cell (Fig. ) consists of eight corner-sharing CoSb6 octahedra surrounding large cavities located at the corners and the centre of the cell (Fig. ). The optimised lattice constant of 𝑎 0 = 8.952 Å obtained using PBEsol + 𝑈 is within 1 % of typical experimental measurements of 9.035-9.039 Å. The calculated phonon dispersion and atom-projected density of states of CoSb3 are shown in Fig. .