Table 1. Experimental Details and Parameters of Synthesis with Different Temperatures^a.

temperature	130 °C	150 °C	170 °C
Pb:Sn (molar ratio)	1:2.5	1:2.5	1:2.5
time (s)	5	5	5
PL peak (nm)	499	503	503
FWHM	23.7	21.2	22.6
PLQY (%)	65.0 ± 0.7	73.4 ± 0.9	69.8 ± 1.3
Bandgap (eV)	2.45	2.43	2.39
Stokes shift	12 nm	10 nm	6 nm
	61.2 meV	50.0 meV	29.8 meV
XRD	*/#	*/#	*/#
Average Sn (%)	18.8 ± 1.0	7.0 ± 2.0	0
Average size (nm)	9 ± 6	6.2 ± 0.5	12 ± 3

^aIn all of the cases, the reaction time (5 s) and the precursor amount and ratio were fixed. The amount of PbBr₂ precursor in the synthesis was 0.166 mmol, and the amount of precursor SnBr₂ was 2.5 times higher. The amount of Cs-oleate was 0.045 mmol. Optical properties of the synthesized NPs have been measured on solution after synthesis and after washing allowing the following to be determined: the PL peak position, the full width at half-maximum (FWHM) of the PL peak, the PLQY, the bandgap extracted from the analysis of the Tauc plots of the absorption measurements, and finally the Stokes shift between light absorption and emission. XRD indicates the crystalline compounds detected by XRD, see Figure 1a, # = CsXBr₃ and * = Cs₄XBr₆, where X is Pb and Sn, the % of Sn measured by EDX during the TEM characterization is also indicated. The average size of the nanoparticles obtained by the statistical analysis of TEM images is also indicated; more detailed histograms with NP size can be found in Figure S6.