Reduced multiplicity of crystallographic sites for superior thermoelectric performance of cubic Cu6GeTeS4 via chemical tailoring.

Argyrodites have garnered substantial interest as ultralow lattice thermal conductivity (κlat) materials. However, their development is constrained by poor thermal stability and inferior thermoelectric performance. Here, a cubic argyrodite compound, Cu6GeTeS4, 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 Cu8GeS6, which undergoes a phase transition from orthorhombic to cubic. The weak Cu─S/Te chemical bonds and cage-like [TeCu18]16+ vibrations at low frequencies lead to intrinsically ultralow κlat ~ 0.47 to 0.35 W m-1 K-1 and realize the maximum ZT (ZTmax) of ~0.62 for Cu6GeTeS4. By introducing S deficiencies, the Cu6GeTeS3.82 obtained an excellent peak ZTmax of 1.23 at 925 K with an ultralow κlat of ~0.25 W m-1 K-1. This study presents high-performance thermoelectrics and provides insights to the design of liquid-like systems.