Rattling vibrations and occupied antibonding states yield intrinsically low thermal conductivity of the Zintl-phase compound KSrBi

Zintl-phase compounds garner attention as promising thermoelectric materials due to observations of phonon-glass electron-crystal (PGEC) behavior, in combination with tunability that allows optimization of properties and doping. However, this is very much dependent on the specific materials, and understanding the factors that lead to PGEC behavior in some Zintl compounds but not others is an important challenge. Here, we investigate KSrBi and SrLiBi. KSrBi exhibits a significantly lower lattice thermal conductivity than SrLiBi, with the 300 K thermal conductivity of KSrBi (0.7 W/mK) being only one third of that in SrLiBi (2.2 W/mK). We find that pronounced rattling behavior of the K atoms in KSrBi leads to strong anharmonicity. The behavior of the two compounds is distinct due to the presence of Sr atoms within the cagelike structure formed by Li and Bi atoms in SrLiBi. The resulting enhanced bonding interactions between Li and Bi weaken the rattling vibrations of Li atoms in SrLiBi, hence influencing its thermal conductivity. Conversely, in KSrBi, K atoms reside within a framework formed by Sr and Bi atoms, exhibiting significant rattling vibrational behavior within this framework. This behavior results in strong scattering of heat-carrying phonons, and in particular large anharmonic scattering rates. Additionally, we find an antibonding electronic state involving the Bi $6p$ orbital and Sr $4p$ orbitals around the valence band edge in KSrBi but not SrLiBi. These antibonding states significantly weaken the bonding, resulting in a softer lattice and reduced sound velocity. Consequently, the combined effects of the strong rattling vibrations of K atoms and the presence of occupied antibonding states near the valence band maximum contribute to a lower lattice thermal conductivity in KSrBi relative to SrLiBi.