Overdamped Phonon Diffusion and Nontrivial Electronic Structure Leading to a High Thermoelectric Figure of Merit in KCu<sub>5</sub>Se<sub>3</sub>
Fan Li, Xin Liu, Ni Ma, Yi‐Chang Yang, Jianping Yin, Ling Chen, Li‐Ming Wu
Abstract
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.