Ultralow Glassy Thermal Conductivity and Controllable, Promising Thermoelectric Properties in Crystalline <i>o</i>-CsCu₅S₃

Abstract

We thoroughly investigated the anharmonic lattice dynamics and microscopic mechanisms of the thermal and electronic transport characteristics in orthorhombic <i>o</i>-CsCu₅S₃ 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 ∼<i>T</i>^-0.33. These findings agree well with experimental values along with the parallel to the Bridgman growth direction. The κ_L in <i>o</i>-CsCu₅S₃ is suppressed down to the amorphous limit, primarily due to the unconventional Cu-S bonding induced by the <i>p-d</i> 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 <i>p-d</i> 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 <i>o</i>-CsCu₅S₃. Our work not only elucidates the anomalous electronic and thermal transport behavior in the copper-based chalcogenide <i>o</i>-CsCu₅S₃ but also provides insights for manipulating its thermal and electronic properties for potential thermoelectric applications.