RbBi2 CsBi2 γ (mJ mol -1 K -2 ) 5.0(4) 4.9( ) The electronic band structure and electronic density of states (DOS) for both compounds are shown in Fig. . The band structure around the Fermi level is similar in both cases. The Fermi level lies within a peak of DOS, thus even a small difference of band structure detailed below, lead to higher DOS at Fermi level in case of CsBi2 (DOS(EF) = 1.51 eV -1 /f.u) than RbBi2 (DOS(EF) = 1.31 eV -1 /f.u), which is correlated with a higher transition temperature Tc, as discussed below. The plot of DOS shows that the p-states of Bi contribute the majority of the total DOS around Fermi level, being only slightly hybridized with s states of Rb/Cs. Bi 6s states are positioned well below the EF due to (scalar) relativistic effects . Thus, both the metallic state and superconductivity in ABi2 arises from interacting 6p states of Bi atoms. In the energy range from -2.5 to 3 eV (with respect to EF) the shape of DOS is similar to the one of elemental bismuth. Particularly the minimum at -0.8 eV is similar to the one at Fermi level of pure Bi. Around the EF the DOS is similar to the case of elemental Bi under pressure p = 2.7 GPa, at which a phase transition to a monoclinic structure is observed and Bi becomes superconducting below Tc = 3.9 K . In this sense, the presence of Rb/Cs atoms impose a chemical pressure on the lattice of Bi atoms, leading to stronger 6p orbital overlap and metallic bonding of the Bi framework.