When we consider aggregates of Sb and Bi, see Figure , we observe a decrease to 2.59 and 2.71 eV of the indirect and direct band gaps, respectively, see Figure . In terms of stability, the two models show the same energy suggesting that both appear equally in the crystal structure. This clearly suggests that the decreasing of the band gap experimentally found for the mixed Sb/Bi material is associated to the presence of this kind of aggregates. In fact, considering the energy alignment in the density of states, Figure , we find an upshift of the VB and CB moving from Cs3Bi2Br9 to Cs3SbBiBr9 of 0.52 eV and of 0.15 eV, respectively. In The band gap bowing of mixed metal perovskites has been observed in Pb/Sn perovskites and attributed to chemical effects mismatch in energy between s and p atomic orbitals of the two metals. A similar interpretation holds also the present Cs3(Sb1-xBix)2Br9 b)