Influence of Cationic Ordering on the Lattice Dynamics\nof Monoclinic Cu<sub>5</sub>Sn<sub>2</sub>S<sub>7</sub> and Cubic\nCu<sub>5</sub>Sn<sub>2</sub>S<sub>6.65</sub>Cl<sub&g

Cu<sub>2</sub>SnS<sub>3</sub> is the parent compound of a series\nof phases in the Cu<sub>2+<i>x</i></sub>Sn<sub>1–<i>x</i></sub>S<sub>3</sub> section (0 ≤ <i>x</i> ≤ 0.15) of the ternary Cu–Sn–S phase diagram,\nthe crystal structure of which can be controlled by varying the synthesis\nprocess and/or through a fine-tuning of the chemical composition.\nDespite being structurally close to the sphalerite structure, the\nthermal transport of these compounds is strongly dependent on the\nexact lattice symmetry and degree of atomic disorder. Here, we investigate\nthe lattice dynamics of the monoclinic ordered Cu<sub>5</sub>Sn<sub>2</sub>S<sub>7</sub> (space group <i>C</i>2) and cubic\ndisordered Cu<sub>5</sub>Sn<sub>2</sub>S<sub>6.65</sub>Cl<sub>0.35</sub> (space group <i>F</i>4̅3<i>m</i>) compounds\nby temperature-dependent powder inelastic neutron scattering (INS).\nIn both cases, the INS spectra feature low-energy optical modes mostly\nweighed by the thermal motion of Cu atoms. The response of the INS\nspectra of Cu<sub>5</sub>Sn<sub>2</sub>S<sub>6.65</sub>Cl<sub>0.35</sub> to temperature variations is indicative of quasi-harmonic behavior.\nCombined with analyses of the low-temperature specific heat, these\nfindings show that the significant lowering of the lattice thermal\nconductivity in cubic Cu<sub>5</sub>Sn<sub>2</sub>S<sub>6.65</sub>Cl<sub>0.35</sub> is due to a reduced phonon mean free path tied\nto the increased level of disorder in the unit cell. These results\nhighlight how the stabilization of highly symmetric, yet strongly\ndisordered crystal structures akin to those observed in high-entropy\nalloys, can lead to a drastic reduction in the heat transport, offering\nan effective approach to design high-performance thermoelectric sulfides.