  Ternary intermetallic half-Heusler (HH) compounds (XYZ) with 18 valence
electron count viz. ZrCoSb, ZrNiSn, and ZrPdSn, have revealed promising
thermoelectric properties. Exemplarily, it has been experimentally observed
that a slight change in the content of Y-site atoms (by ~3-12.5% i.e., m =0.03,
0.125 in ZrY(1+m)Z) leads to drastic lowering in the lattice thermal
conductivity (kL) by more than 65-80% in many of these compounds. The present
work aims at exploring the possibility of maximizing the electronic transport
scenario after achieving the low kL limit in these compounds. By taking into
account the full anharmonicity of the lattice dynamics, Boltzmann transport
calculations are performed under the framework of density functional theory.
Our results show that these excess atoms present in the vacant lattice site
induce scattering by acting either as a rattling mode or by hybridizing with
the acoustic modes of the host depending upon their mass and bonding chemistry,
respectively. Furthermore, the introduction of these scattering centers may
lead to the formation of a defect mid-gap state in the electronic band
structure (detrimental for electronic transport) or lead to light doping of the
host compound. The latter is found to be particularly conducive for attaining
synergy in both thermal as well as electronic transport.
