Large thermoelectric effect driven by high-order anharmonicity from synergistic lone-pair electrons and rattling modes in K3Au3Sb2

Abstract

Using first-principles calculations, self-consistent phonon theory and Boltzmann transport equation, we investigate the effect of high-order anharmonicity on the thermal transport and thermoelectric properties of K3Au3Sb2. Through examination of atomic vibration modes, structural anisotropy and analysis of three- and four-phonon scattering processes, it is found that lattice thermal conductivity κ_L as low as ~0.5 Wm-1K-1 is achieved at room temperature. Our study reveals that the intrinsic ultralow lattice thermal conductivity (κ_L) is primarily attributed to the pronounced quartic anharmonicity of the heat-carrying Au atoms. This anharmonicity is rooted in the intrinsically soft lattice features and is further enhanced by the asymmetric potential energy surface induced by Sb lone-pair electrons, alongside their synergistic coupling with the Au-Sb covalent bonds. Furthermore, the K2 atoms located between cationic layers exhibit intense rattling modes; their coupled vibrations with the Au-Sb anionic framework further intensify phonon scattering and suppress κ_L, as evidenced by the presence of avoided crossing points between the longitudinal acoustic (LA) and low-frequency optical branches. Regarding electronic transport, the system shows anisotropy identical to that in thermal transport. After accounting for spin-orbit coupling (SOC) effects, although relativistic impacts exert a certain degree of modulation on the electronic parameters, the system maintains exceptional thermoelectric performance at 900 K: the peak ZT values reach 4.17 for p-type (c-axis) and 2.96 for n-type (a(b)-axis). This work reveals the synergy between lone-pair-induced high-order anharmonicity and guest atom rattling in suppressing thermal transport, providing important insights for achieving superior thermoelectric performance in Zintl phase compounds.