In order to better describe the difference between the phonon structures of CsBi2 and RbBi2, dispersion relations around the Г point are plotted and the modes associated with similar movement of atoms are marked with the same color in Fig. of the Supplementary Material. Additionally, modes associated with the movement of Bi atoms are visualized in Fig. ) of the Supplementary Material and force constants between atoms are shown in Fig. (g). The modes associated with a movement of both Bi (along Bi-Bi bonding) and Cs/Rb (Fig. ) are lowered from 1.31 in case of RbBi2 to 1.03 THz in CsBi2, while the frequency of modes associated with a movement of Bi towards the empty space around Cs/Rb (Fig. )) are increased from 1.22 THz and 1.32 THz in RbBi2 to 1.36 THz in CsBi2. In both compounds the phonon linewidths are large only in case of Bi modes. Since the FS is also dominated by 6p-states of Bi, the superconductivity in ABi2 is driven by the Bi pyrochlore lattice, as the electron-phonon coupling in both electronic and phonon ways is almost exclusively contributed by Bi.