Effect of multinary substitution on electronic and transport properties of TiCoSb based half-Heusler alloys

TL;DR

This study investigates how multinary substitution in TiCoSb-based half-Heusler alloys affects their electronic structure and thermoelectric properties, revealing potential improvements in thermoelectric efficiency through specific elemental substitutions.

Contribution

It provides a detailed analysis of how substituting Zr, Hf, Pb, and Te alters the band structure and thermoelectric performance of TiCoSb alloys, highlighting the role of Hf in enhancing ZT at room temperature.

Findings

Substitutions lower the band gap and change it from indirect to direct.

Hf substitution improves the ZT value (~1.05) at room temperature.

No significant ZT difference at higher temperatures with Hf substitution.

Abstract

The electronic structures of TixZrx/2CoPbxTex, TixZrx/2Hfx/2CoPbxTex (x = 0.5), and the parent compound TiCoSb were investigated using the full potential linearized augmented plane wave method. The thermoelectric transport properties of these alloys are calculated on the basis of semi-classical Boltzmann transport theory. From the band structure calculations we show that the substitution of Zr,Hf in the Ti site and Pb and Te in the Sb site lower the band gap value and also change the indirect band (IB) gap of TiCoSb to the direct band (DB) gap. The calculated band gap of TiCoSb, TixZrx/2CoPbxTex, and TixZrx/2Hfx/2CoPbxTex are 1.04 eV (IB), 0.92 eV (DB), and 0.93 eV (DB), respectively. All these alloys follow the empirical rule of 18 valence-electron content which is essential for bringing semiconductivity in half Heusler alloys. It is shown that the substitution of Hf at the Ti site improve the ZT value (~1.05) at room temperature, whereas there is no significant difference in ZT is found at higher temperature. Based on the calculated thermoelectric transport properties, we conclude that the appropriate concentration of Hf substitution can further improve the thermoelectric performance of TixZrx/2Hfx/2CoPbxTex.