Significant first-principles electron-phonon coupling effects in the LiZnAs and ScAgC half-Heusler thermoelectrics

TL;DR

This study uses first-principles calculations to analyze electron-phonon interactions in LiZnAs and ScAgC half-Heusler thermoelectrics, revealing high thermoelectric efficiency due to phonon-induced electron scattering and nanostructuring.

Contribution

It provides detailed first-principles insights into electron-phonon effects and thermoelectric performance in half-Heusler compounds, highlighting the impact of nanostructuring on $zT$.

Findings

LiZnAs achieves $zT$ of 1.05 at 900 K; nanostructuring increases it to 1.53.

ScAgC reaches $zT$ of 0.78; nanostructuring increases it to 1.0.

Electron-phonon interactions significantly influence thermoelectric properties.

Abstract

The half-Heusler (hH) compounds are currently considered promising thermoelectric (TE) materials due to their favorable thermopower and electrical conductivity. Accurate estimates of these properties are therefore highly desirable and require a detailed understanding of the microscopic mechanisms that govern transport. To enable such estimations, we carry out comprehensive first-principles computations of one of the primary factors limiting carrier transport, namely the electron-phonon ($e-ph$) interaction, in LiZnAs and ScAgC. Our study first investigates their electron and phonon dispersions and then examines the temperature-induced renormalization of the electronic states. We then solve the Boltzmann transport equation (BTE) under multiple relaxation-time approximations (RTAs) to evaluate the carrier transport properties. Phonon-limited electron and hole mobilities are comparatively assessed using the linearized self-energy and momentum RTAs (SERTA and MRTA), and the exact or iterative BTE (IBTE) solutions within $e-ph$ coupling. Electrical transport coefficients for TE performance are also comparatively analyzed under the constant RTA (CRTA), SERTA, and MRTA schemes. The lattice thermal conductivity, determined from phonon-phonon interaction, is further reduced through nanostructuring techniques. The bulk LiZnAs (ScAgC) compound achieves the highest figure of merit ($zT$) of 1.05 (0.78) at 900 K with an electron doping concentration of 10$^{18}$ (10$^{19}$) cm$^{-3}$ under the MRTA scheme. This value significantly increases to 1.53 (1.0) for a 20 nm nanostructured sample. The remarkably high $zT$ achieved through inherently present phonon-induced electron scattering effects, combined with grain-boundary engineering, opens a promising path for discovering highly efficient and accurate next-generation hH TEs.