This may be caused by a difference in the unit cell size, which is larger in the case of CsBi2, thus the Bi-Bi metallic bonding is slightly weaker leading to smaller force constants of Bi-Bi pairs (see Fig. (h) of the Supplementary Material) and lower frequencies of modes associated with a movement of Bi along Bi-Bi bonds. For the same reason, the highest optical mode associated with a movement of Bi toward the center of tetrahedron (Fig. (f)) is lowered from 2.37 THz in case of RbBi2 to 1.78 THz in CsBi2. It is similar to the case of soft mode shown in Fig. (e) and explains why the effect of softening is stronger in case of CsBi2. On the other hand, the force constants of Bi-Cs pairs are larger than of Bi-Rb pairs, leading to a higher frequencies of modes associated with a movements of Bi toward the space around Cs. These changes show that the substitution of Rb by Cs not only lowers the frequency due to a larger mass, but also influences the size of the unit cell and modifies the bonding of Bi network.