The measured absorption spectra of the bromide and iodide alloys, together with the pure titanium and tin spectra, are shown in Figure (a) and (d), respectively. A key difference between the bromide and the iodide compounds is that, while the difference in bandgap between Cs 2 TiBr 6 and Cs 2 SnBr 6 is ~1.1 eV, the bandgap difference between Cs 2 TiI 6 and Cs 2 SnI 6 is only ~0.3 eV, and thus shows a smaller variation in the absorption onset wavelength. From the bromide absorption spectra, the samples containing up to 60% of tin still show the characteristic excitonic peaks of the TiBr 6 -2 octahedra in Cs 2 TiBr 6 , and only when 90% of titanium is replaced by tin these peaks are not visible anymore. The same behavior is witnessed for the iodide alloys, where the excitonic peaks of the TiI 6 -2 octahedra in Cs 2 TiI 6 remain visible at the highest mixed Sn fraction of 82%. Consequently, the absorption onset energy remains effectively constant as Sn 4+ is mixed into the Ti-based compounds, all the way up to alloys with a high Sn +4 mixing percentage (>80%), until the low energy Ti excitonic peaks disappear.