Reduced multiplicity of crystallographic sites for superior thermoelectric performance of cubic Cu 6 GeTeS 4 via chemical tailoring

Argyrodites have garnered substantial interest as ultralow lattice thermal conductivity (κ lat ) materials. However, their development is constrained by poor thermal stability and inferior thermoelectric performance. Here, a cubic argyrodite compound, Cu 6 GeTeS 4 , was successfully synthesized via a chemical-tailoring strategy. Moreover, a strong correlation between the multiplicity of crystallographic sites and ion mobility was found. The reduced multiplicity of Cu crystallographic sites leads to superior thermal stability compared to Cu 8 GeS 6 , which undergoes a phase transition from orthorhombic to cubic. The weak Cu─S/Te chemical bonds and cage-like TeCu 18 16+ vibrations at low frequencies lead to intrinsically ultralow κ lat ~ 0.47 to 0.35 W m −1 K −1 and realize the maximum ZT ( ZT max ) of ~0.62 for Cu 6 GeTeS 4 . By introducing S deficiencies, the Cu 6 GeTeS 3.82 obtained an excellent peak ZT max of 1.23 at 925 K with an ultralow κ lat of ~0.25 W m −1 K −1 . This study presents high-performance thermoelectrics and provides insights to the design of liquid-like systems.