Powdered samples of the Cs3(Sb1-xBix)2Br9 system for x=0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, and 1 have been prepared by using a planetary ball miller according to the experimental conditions reported in the Experimental Section (see the Supporting Information, SI). Figure shows the appearance of the eight samples prepared from Cs3Sb2Br9 (left) to Cs3Bi2Br9 (right). Figure reports the room temperature (RT) x-ray diffraction (XRD) patterns collected on all the samples reported in Figure . Samples have been measured both with Cu-K radiation (Figure ) and with Mo-K radiation (Figure ) to provide the most reliable structural results on these novel samples. Compositions (expressed as x) in Figure refer to the effective stoichiometries determined by microprobe analysis (see later in the text). All the samples are single phase and are in agreement with the hexagonal symmetry of the P-3m1 space group describing the two end members, namely Cs3Sb2Br9 and Cs3Bi2Br9. A superposition of the reference structure model for these last two samples and the experimental patterns are shown in Figures and, respectively. According to these results, the mechanochemical synthetic procedure was effective in preparing single-phase materials for both the two stoichiometric compositions Cs3Sb2Br9 and Cs3Bi2Br9, and for the mixed Sb/Bi samples. Such green and sustainable synthetic approach has never been applied to Sb-based defective perovskites nor to B-site mixed compositions, while a previous report on its effective use for the preparation of Cs3Bi2Br9 is reported in the current literature. The morphology and chemical composition of the samples has been determined by means of scanning electron microscopy (SEM) and microprobe analysis. Images of the sample morphology are reported in Figure . As a consequence of the mechanochemical synthesis, the samples do not possess a well-defined morphology, as occurs in solution chemistry synthesis, and grains with various shapes and dimensions (in the micron and sub-micron range) are observed. Elemental analysis by SEM confirmed a good agreement with the nominal compositions (see Table ). In the rest of the paper, we will make use of the effective compositions instead of the nominal ones when discussing the experimental results.