Self Doping for Synergistically Tuning the Electronic and Thermal Transport Coefficients in n-type Half-Heuslers

TL;DR

This study investigates how self-doping in n-type half-Heusler compounds can optimize thermoelectric performance by reducing thermal conductivity and enhancing electronic transport through atom substitution effects.

Contribution

It introduces a comprehensive theoretical analysis of self-doping effects on thermal and electronic transport in half-Heusler compounds using first-principles calculations.

Findings

Excess atoms induce phonon scattering, reducing lattice thermal conductivity by over 65-80%.

Self-doping can create defect states or light doping, influencing electronic transport.

Synergistic improvement in thermoelectric properties achieved through defect engineering.

Abstract

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.