## 3 Results and Discussion
### Electrical and Optical Properties, Raman Scattering and DFT Calculations
Similar results have been observed for CuCr\({}_{1\text{-}}\)1 Sn\({}_{0\text{-}}\)9S\({}_{4}\) (-5.0 S/cm) and CuCr\({}_{1\text{-}}\)2 Sn\({}_{0\text{-}}\)8S\({}_{4}\) (-40 S/cm) thiospinels [23]. [PERSON] et al. reported large magnetoresistance in CuCrZrS\({}_{4}\). In this phase, the chromium site was partially substituted by a diamagnetic metal. This solid solution exhibited VRH behavior and resistivity that decreased with an increasing magnetic field [21]. In our systems, very low magnetoresistance was observed at an applied magnetic field of 0.556 T (see Table 3). This suggests that the electronic transport properties of CuCrMX4 thio- and selenospinels are highly sensitive to chemical substitution. These materials showed behavior typical of semiconductors, and the band gaps were measured using UV-vis-IR NIR diffuse reflectance spectroscopy (Figure 3). The observed band gaps of CuCr\({}_{2\text{-}}\)5 Sn\({}_{\text{x}}\)5\({}_{2\text{-}}\)3 Fe\({}_{1\text{-}}\)7 and CuCr\({}_{2\text{-}}\)5 Sn\({}_{\text{x}}\)5\({}_{1\text{-}}\)7 Se\({}_{2\text{-}}\)3 ranged from \(\sim\)1.6 to 1.9 eV. These values are comparable to those observed for CoCr\({}_{2}\)S\({}_{4}\) nanocrystals (1.7 eV) and CuCr\({}_{2}\)Se\({}_{4}\) (\(\sim\)2.0 eV) [37; 38]. Raman spectra of the CuCr\({}_{2\text{-}}\)5 Sn\({}_{\text{x}}\)5\({}_{2\text{-}}\)3 Se\({}_{1\text{-}}\)7 and CuCr\({}_{2\text{-}}\)5 Sn\({}_{\text{x}}\)5\({}_{1\text{-}}\)7 Se\({}_{2\text{-}}\)3 materials were compiled inside the 100 to 450 cm\({}^{-1}\) range (Figure 3). The spectra present five signals at approximately \(\sim\)124, 203, 265, 295, and 350 cm\({}^{-1}\). These signals are equivalent to the (Cu)\({}_{\text{tet}}\)[Cr\({}_{2\text{-}}\)5 Sn\({}_{\text{x}}\)]\({}_{\text{oct}}\)5\({}_{2\text{-}}\)2 Se\({}_{2}\) (0.2 \(\leq\) x \(\leq\) 1.0) spinels, where a partial substitution of Cr by Sn occurs [12]. The frequencies and vibrational assignment of Raman bands with respect to the CuCrSnS\({}_{4}\) and (Cu)\({}_{\text{tet}}\)[Cr\({}_{2\text{-}}\)5 Sn\({}_{\text{x}}\)]\({}_{\text{oct}}\)5\({}_{2\text{-}}\)2 Se\({}_{2}\) is shown in Table S2 (see Supplementary Materials). The signals at \(\sim\)350 cm\({}^{-1}\) and \(\sim\)270 cm\({}^{-1}\) have been assigned to the A\({}_{\text{1g}}\) and F\({}_{\text{2g}}\) (2) modes which are associated with symmetrical and asymmetrical stretching of the chalcogen bond in the tetrahedral metal, respectively. We observed a significant shift at lower frequencies for peaks at \(\sim\)350 cm\({}^{-1}\), 295 cm\({}^{-1}\), and \(\sim\)265 cm\({}^{-1}\). The displacement of the signal is attributed to the incorporation of Sn atoms in an octahedral position. The greater weight of Sn than Cr leads to a shift towards lower energies. On the  
Figure 3: (**a**) Representative plot of diffuse reflectance against photon energy (_hv_) for CuCr\({}_{1\text{-}}\)5 Sn\({}_{1\text{-}}\)5S\({}_{2\text{-}}\)3 Se\({}_{1\text{-}}\)7 sample. Red line is included to show the baseline, and linear extrapolation used to determine the band gap. (**b**) Representative Raman spectral of CuCr\({}_{1\text{-}}\)6 Sn\({}_{0\text{-}}\)4S\({}_{2\text{-}}\)3 Se\({}_{1\text{-}}\)7 powder sample with the different contributions deduced from the fitting of the different peaks with Lorentzian curves (green lines). other hand, as Sn-chalcogen interactions influence the environment, this results in a change in the polarities of the bonds inducing variations in the intensity and position of the signals. To rationalize the physical measurements, DFT calculations were carried out on compounds with different elemental compositions. To reach the stoichiometry of the current compounds under discussion, the structure of the CuCr\({}_{2}\)Se\({}_{4}\) spinel was modified to account for the different Cr/Sn and S/Se ratios. Ions were placed randomly, and geometric optimization was repeated for three different ion arrangements for each case.