Ultralow Glassy Thermal Conductivity and Controllable, Promising Thermoelectric Properties in Crystalline o -CsCu 5 S 3 .

We thoroughly investigated the anharmonic lattice dynamics and microscopic mechanisms of the thermal and electronic transport characteristics in orthorhombic o -CsCu5 S3 at the atomic level. Taking into account the phonon energy shifts and the wave-like tunneling phonon channel, we predict an ultralow κL 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 κL in o -CsCu5 S3 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 κL 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 -CsCu5 S3 . Our work not only elucidates the anomalous electronic and thermal transport behavior in the copper-based chalcogenide o -CsCu5 S3 but also provides insights for manipulating its thermal and electronic properties for potential thermoelectric applications.