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, 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 κ_lat ~ 0.47 to 0.35 W m^-1 K^-1 and realize the maximum <i>ZT</i> (<i>ZT</i>_max) of ~0.62 for Cu₆GeTeS₄. By introducing S deficiencies, the Cu₆GeTeS_3.82 obtained an excellent peak <i>ZT</i>_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.