Abstract
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.