Optical phonon modes assisted thermal conductivity in p-type ZrIrSb Half-Heusler alloy: A combined experimental and computational study

TL;DR

This study combines experimental and computational methods to explore how optical phonon modes influence thermal conductivity in p-type ZrIrSb Half-Heusler alloy, revealing mechanisms that could improve thermoelectric performance.

Contribution

It provides new insights into phonon interactions and thermal transport in heavy-element Half-Heusler alloys, highlighting the role of optical phonons in reducing thermal conductivity.

Findings

Coupling between acoustic and optical phonons lowers lattice thermal conductivity.

Anti-site disorder affects electrical properties near room temperature.

Heavy atoms induce optical phonon modes that enhance Umklapp scattering.

Abstract

Half Heusler (HH) alloys with 18 valence electron count have attracted significant interest in the area of research related to thermoelectrics. Understanding the novel transport properties exhibited by these systems with semiconducting ground state is an important focus area in this field. Large thermal conductivity shown by most of the HH alloy possesses a major hurdle in improving the figure of merit (ZT). Additionally, understanding the mechanism of thermal conduction in heavy constituents HH alloys is an interesting aspect. Here, we have investigated the high temperature thermoelectric properties of ZrIrSb through experimental studies, phonon dispersion and electronic band structure calculations. ZrIrSb is found to exhibit substantially lower magnitude of resistivity and Seebeck coefficient near room temperature, owing to existence of anti-site disorder between Ir/Sb and vacant sites. Interestingly, in ZrIrSb, lattice thermal conductivity is governed by coupling between the acoustic and low frequency optical phonon modes, which originates due to heavier Ir/Sb atoms. This coupling leads to an enhancement in the Umklapp processes due to the optical phonon excitations near zone boundary, resulting in a lower magnitude of \k{appa}L. Our studies point to the fact that the simultaneous existence of two heavy mass elements within a simple unit cell can substantially decrease the lattice degrees of freedom.