Zintl chemistry leading to ultralow thermal conductivity, semiconducting behavior, and high thermoelectric performance of hexagonal KBaBi

We identify the ground state structure of the phase KBaBi using ab initio evolutionary structure search methods and demonstrate that the bonding of this compound leads to a combination of very low thermal conductivity and electronic properties that are favorable for thermoelectric performance. The structure is the hexagonal ZrBeSi-type structure, with the cubic half-Heusler structure as a higher-energy competing phase. The bonding of the hexagonal structure leads to strongly anharmonic vibrational properties, which underlie the low thermal conductivity. The calculated figure of merit is approximately $ZT=3$ at 1000 K with optimized doping. This work underscores the use of chemical control of bonding to obtain low thermal conductivity and identifies a chemistry that achieves this.