  Half-Heusler compounds usually exhibit relatively higher lattice thermal
conductivity that is undesirable for thermoelectric applications. Here we
demonstrate by first-principles calculations and Boltzmann transport theory
that the BiBaK system is an exception, which has rather low thermal
conductivity as evidenced by very small phonon group velocity and relaxation
time. Detailed analysis indicates that the heavy Bi and Ba atoms form a
cage-like structure, inside which the light K atom rattles with larger atomic
displacement parameters. In combination with its good electronic transport
properties, the BiBaK shows a maximum n-type ZT value of 1.9 at 900 K, which
outperforms most half-Heusler thermoelectric materials.
