Unravelling the self doping induced lowering in the lattice thermal conductivity of n-type thermoelectric half Heusler compounds

Ternary intermetallic half Heusler (HH) compounds (XYZ) with 18 valence electron count viz. ZrNiSn, ZrPdSn, and ZrCoSb, have revealed interesting thermoelectric properties pertaining to their narrow bandgap and flexibility in undergoing doping. Exemplarily, it has been experimentally observed that a slight change in the content of Y-site atoms (by ~3% i.e. n =0.03 in ZrY_1+nZ) leads to drastic lowering in the lattice thermal conductivity (k_L) by more than ~ 60 % in all these compounds [1]. Thus, it is interesting to explore the impact of these off stoichiometric defects on the transport coefficients and the underlying physical mechanisms. We have performed the anharmonic lattice dynamics calculations using the Boltzmann transport equation under the framework of density functional theory to analyze the ongoing phenomena. It reveals that the excess atoms present at the vacant sites as interstitial defects act as phonon scattering center and enable strong scattering of the thermal phonons, resulting in a significant reduction of the k_L. Furthermore, the excess electrons beyond 18 provide substantial doping (n-type) which improves the electronic transport coefficients. Our findings can be extended to other HH compounds.

[1] Chauhan et. al. , Appl. Phys. Lett. 113, 013902 (2018).