Abnormal thermal conduction in argyrodite-type Ag9FeS6-Te materials

Rationalizing the underlying mechanism that correlates phonon transport with structural complexity is crucial for the development of materials with ultralow thermal conductivity. Herein, we investigated the abnormal thermal transport and lattice dynamics in Ag9FeS6-xTex argyrodite-type compounds via a combination of properties characterization, model analysis, and theoretical calculations. Our results show that the ordered α-phase and disordered β-phase of Ag9FeS6-xTex exhibit distinct temperature dependency of lattice thermal conductivity κL. Specifically, the κL of ordered α-phase shows a crystalline characteristic with a pronounced Umklapp peak observed at ∼10 K, while the κL of disordered β-phase monotonously increases in the whole temperature range without a peak or plateau. Such different thermal transport behavior is a result of different contributions to the reduction of κL from the structural disorder and the low-lying multi-Einstein oscillators, both of which are associated with the weakly bonded Ag+ ions. The knowledge depicted here provides fundamental insights into the extraordinary thermal transports in materials with structural disorder and/or weak chemical bonds.