High-Performance Thermoelectric Properties of Half-Heusler CoHfSi: A First-Principles Study with Temperature-Dependent Relaxation Time

Abstract: In the ongoing search for innovative thermoelectric (TE) materials with superior TE performance globally, we aim to investigate the possible use of half-Heusler alloy CoHfSi in TE applications. We analyzed the structure stability, thermodynamic inertia and electrical and thermal transport properties using density functional formalism and semi-classical Boltzmann transport theory. Positive phonon frequencies confirm this alloy's dynamical stability, and the Born-Huang stability criterion is also satisfied, confirming the robust mechanical stability. A large Seebeck voltage of more than 150 {\mu}V/K is estimated, an essential and typical requirement for improved heat-to-electricity conversion efficiency. This Seebeck voltage can be further increased by an order of magnitude with suitable doping. The PHONO3PY algorithm and Slack's model are used to compare the lattice thermal conductivity. The latter method gives more values than the former algorithm. Despite the commonly used constant relaxation time approximation to estimate the TE performance, we adopt the temperature-dependent relaxation time and found a clear drop in figure-of merit (zT) from those estimated without considering the lattice thermal conductivity and relaxation time both, still, the zT values are remarkably more than two, for the temperatures above 500 K, which is a striking numeral in the field of TE materials.