Soft Phonon Modes Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance in AgCuTe

Crystalline solids exhibiting intrinsically low lattice thermal conductivity (κL) are crucial to realizing high-performance thermoelectric (TE) materials. Herein, we demonstrate an ultralow κL of 0.35 Wm-1K-1 in AgCuTe, which exhibits a remarkable TE figure-of-merit, zT of 1.6 at 670 K when alloyed with 10 mol% Se. First-principles DFT calculation reveals the presence of several soft phonon modes in its room-temperature hexagonal phase, which are also evident from low-temperature heat capacity measurement. These phonon modes, dominated by Ag vibrations, soften further with temperature giving a dynamic cation disorder and plausibly driving the superionic transition. Intrinsic factors such as hierarchical bond strengths, soft modes, and optical-acoustic phonon coupling cause an ultralow κL in the room-temperature hexagonal phase, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high-temperature rocksalt phase. Despite the significant cation disorder at elevated temperatures, the crystalline conduits to carrier-transport provided by the rigid anion sublattice render a high power factor.