Overdamped Phonon Diffusion and Nontrivial Electronic Structure Leading to a High Thermoelectric Figure of Merit in KCu₅Se₃

Thermoelectric copper selenides are highly attractiveowing tonot only their constituent nontoxic, abundant elements but also theirultralow liquid-like lattice thermal conductivity (& kappa;(lat)). For the first time, the promising thermoelectric properties ofthe new KCu5Se3 are reported herein, showinga high power factor (PF = 9.0 & mu;Wcm(-1) K-2) and an intrinsically ultralow & kappa;(lat) = 0.48 Wm(-1) K-1. The doped K1-x Ba x Cu5Se3 (x = 0.03) realizes a figure-of-meritZT = 1.3 at 950 K. The crystallographic structure of KCu5Se3 allows complex lattice dynamics that obey a rare dual-phonontransport model well describing a high scattering rate and an extremelyshort phonon lifetime that are attributed to interband phonon tunneling,confinement of the transverse acoustic branches, and temperature-dependentanharmonic renormalization, all of which generate an unprecedentlyhigh contribution of the diffusive phonons (70% at 300 K). The overallweak chemical bonding feature of KCu5Se3 givesK(+) cations a quiescence behavior that further blocks theheat flux transfer. In addition, the valence band edge energy dispersionof KCu5Se3 is quasilinear that allows a largeSeebeck coefficient even at high hole concentrations. These in-depthunderstandings of the ultralow lattice thermal conductivity providenew insights into the property-oriented design and synthesis of advancedcomplex chalcogenide materials.