Interplay between anharmonic lattice dynamics and bipolar-transport-driven anomalous transport in Zintl phase K X Sb ( X = Ge , Sn , Pb )

This study employed first-principles calculations to investigate the thermoelectric transport properties of the KXSb $(X=\mathrm{Ge},\mathrm{Sn},\mathrm{Pb})$ series compounds. Both the harmonic approximation (HA) and self-consistent phonon (SCP) theory were adopted, incorporating multiple phonon scattering mechanisms and spin-orbit coupling (SOC) effects. The results reveal that the chemical bonding characteristics and mechanical properties evolve systematically with increasing atomic number of X. The KXSb series exhibits pronounced lattice anharmonicity. As the atomic mass increases, the phonon spectrum progressively flattens, which, through multiple mechanisms, leads to ultralow lattice thermal conductivity. Three-phonon scattering is identified as the dominant mechanism suppressing lattice thermal conductivity, while four-phonon scattering provides an important additional contribution. In terms of electronic transport, SOC induces significant spin splitting in the band structure, substantially affecting the thermoelectric quality factor. An anomalous sign reversal of the Seebeck coefficient is observed in electrical transport, which originates from the bipolar transport effect associated with the narrow band gap and band anisotropy. Furthermore, the electrical transport performance under $n$-type doping is generally superior to that under $p$-type doping. As a consequence of these combined effects, the KXSb series exhibits excellent thermoelectric performance, with KSnSb achieving a maximum ZT of approximately 3.77 at 800 K.