Overdamped Phonon Diffusion and Nontrivial Electronic Structure Leading to a High Thermoelectric Figure of Merit in KCu 5 Se 3 .

Thermoelectric copper selenides are highly attractive owing to not only their constituent nontoxic, abundant elements but also their ultralow liquid-like lattice thermal conductivity (κ lat ). For the first time, the promising thermoelectric properties of the new KCu 5 Se 3 are reported herein, showing a high power factor (PF = 9.0 μWcm -1 K -2 ) and an intrinsically ultralow κ lat = 0.48 Wm -1 K -1 . The doped K 1- x Ba x Cu 5 Se 3 ( x = 0.03) realizes a figure-of-merit ZT = 1.3 at 950 K. The crystallographic structure of KCu 5 Se 3 allows complex lattice dynamics that obey a rare dual-phonon transport model well describing a high scattering rate and an extremely short phonon lifetime that are attributed to interband phonon tunneling, confinement of the transverse acoustic branches, and temperature-dependent anharmonic renormalization, all of which generate an unprecedently high contribution of the diffusive phonons (70% at 300 K). The overall weak chemical bonding feature of KCu 5 Se 3 gives K + cations a quiescence behavior that further blocks the heat flux transfer. In addition, the valence band edge energy dispersion of KCu 5 Se 3 is quasilinear that allows a large Seebeck coefficient even at high hole concentrations. These in-depth understandings of the ultralow lattice thermal conductivity provide new insights into the property-oriented design and synthesis of advanced complex chalcogenide materials.