Microscopic mechanism of low lattice thermal conductivity induced by strong anharmonic vibrations in thermoelectric zintl compounds KXAs (X = Sn, Ge)

The thermoelectric properties of Zintl compounds K$X$As ($X$ = Sn, Ge) were systematically investigated using first-principles calculations in combination with density functional theory (DFT), self-consistent phonon (SCP) theory, and the Boltzmann transport equation (BTE). Both three-phonon and four-phonon scattering processes were explicitly considered. The results reveal that the lattice thermal conductivity (${\kappa }_L$) remains significantly below 1 W/mK at high temperatures, which is mainly attributed to strong quartic anharmonicity induced by weak bonding between K atoms and their neighboring atoms, together with enhanced Umklapp scattering, effectively suppressing phonon transport. For electronic transport, multiple carrier scattering mechanisms were incorporated to provide a reasonable estimation of the carrier relaxation time. Furthermore, spin-orbit coupling (SOC)-induced Rashba splitting leads to a remarkable reconstruction of the electronic band structure, exerting a pronounced influence on the thermoelectric performance. The significant asymmetry in the band-edge curvature between the conduction and valence bands results in an imbalance of electron and hole contributions, giving rise to an unusual negative Seebeck coefficient under $p$-type doping conditions, thereby challenging the conventional bipolar transport theory. Overall, the maximum thermoelectric figure of merit ($ZT$) along the $c$ axis reaches 2.88 for KSnAs and 2.78 for KGeAs at 800 K, demonstrating excellent thermoelectric performance and broad application potential.