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

Using first-principles calculations, self-consistent phonon theory and the 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 a lattice thermal conductivity (κ_L) as low as ∼0.5 W m^-1 K^-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 induce a certain degree of modulation in the electronic parameters, the system maintains exceptional thermoelectric performance at 900 K: the peak ZT values reach 4.17 for the p-type (c-axis) and 2.96 for the 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.