The Raman investigation has been performed focusing on the low energy part of the Raman activity, i.e. in the range 25-250 cm -1 . At a first glance, the single-phase of the endmembers' samples is confirmed, as reported in Figure . Indeed, the Raman spectrum of the Cs3Bi2Br9 sample clearly shows the characteristic modes at 165 and 190 cm - , due to the bond vibrations inside the octahedral cage, BiBr6, as already described in our previous work. In analogy with the isomorphic Cs3Bi2Br9, also for Cs3Sb2Br9 the Raman response qualitatively presents the same spectral fingerprints: a broadened structure centered at about 70 cm -1 clearly resulting from the overlapping of different modes and two sharp and symmetric modes peaked at 182 and 210 cm -1 , resulting from the Sb-Br bond vibrations in the octahedral unit SbBr6. The proper assignment of the mode symmetry seems to be controversial. The crossover between Bi-rich to Sb-rich system is also well evidenced in Figure where the intensity of the mode at about 165 cm -1 proper of Cs3Bi2Br9 and the one at 210 cm -1 of Cs3Sb2Br9 are plotted as a function of the Bi content: as the former increases, the latest decreases. In Figure , the spectrum for the x = 0.57 sample is reported together with the result from the best-fitting procedure using four Lorentzian curves according to the above mentioned two-mode behavior. observed and described also from a computational point of view. The values of the direct and indirect band gaps determined from the Tauc plots (reported in Figure ) are shown in Figure as a function of x. The values of direct and indirect band gaps for Cs3Sb2Br9 and Cs3Bi2Br9 are in agreement with those previously reported in the current literature. By looking at Figure , a significant bowing of the band gap for mixed compositions is observed. This result is unexpected since, in general, metal ion replacement in solid solution provides a scaling of the band gap obeying the Vegard's law. The reduction of the band gap (direct) for mixed compositions is relevant, reaching a minimum value of about 2.35 eV when x is around 0.2.