Ultralow Glassy Thermal Conductivity and Controllable, Promising Thermoelectric Properties in Crystalline o-CsCu₅S₃

We thoroughly investigated the anharmonic lattice dynamics and microscopic mechanisms of the thermal and electronic transport characteristics in orthorhombic o -CsCu 5 S 3 at the atomic level. Taking into account the phonon energy shifts and the wave-like tunneling phonon channel, we predict an ultralow κ L of 0.42 w/mK at 300 K with an extremely weak temperature dependence following ∼ T –0.33 . These findings agree well with experimental values along with the parallel to the Bridgman growth direction. The κ L in o -CsCu 5 S 3 is suppressed down to the amorphous limit, primarily due to the unconventional Cu–S bonding induced by the p–d hybridization antibonding state coupled with the stochastic oscillation of Cs atoms. The nonstandard temperature dependence of κ L can be traced back to the critical or dominant role of wave-like tunneling of phonon contributions in thermal transport. Moreover, the p–d hybridization of Cu(3)–S bonding results in the formation of a valence band with “pudding-mold” and high-degeneracy valleys, ensuring highly efficient electron transport characteristics. By properly adjusting the carrier concentration, excellent thermoelectric performance is achieved with a maximum thermoelectric conversion efficiency of 18.4% observed at 800 K in p-type o -CsCu 5 S 3 . Our work not only elucidates the anomalous electronic and thermal transport behavior in the copper-based chalcogenide o -CsCu 5 S 3 but also provides insights for manipulating its thermal and electronic properties for potential thermoelectric applications.