When placed in the centre of the cavity, as shown in Fig. , the first, second and third neighbours of the filler atoms are 12 Sb atoms, 8 Co atoms, and a further 24 Sb atoms respectively. The corresponding ‖𝚽‖ increase by an order of magnitude from He to Xe, from 1.78 × 10 -1 , 1.78 × 10 -2 and 4.66 × 10 -3 eV Å -2 in HeCo8Sb24 to 1.63, 2.32 × 10 -1 and 4.88 × 10 -2 eV Å -2 in XeCo8Sb24 (Table .2). The stronger interactions between the larger/heavier filler atoms and the host framework explains the increasingly prominent frequency shifts in the unfolded phonon dispersions. The ‖𝚽‖ for the nearest-neighbour Co-Sb and Sb-Sb interactions in pristine CoSb3 are 4.40 and 5.02 eV Å -2 , respectively, so the first-neighbour Kr-Sb and Xe-Sb interactions are of comparable strength to the bonding in the host framework.