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

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.

Figures (9)

• Figure 1: (a) Primitive unit cell used for the DFT, DFPT and EPI calculations. Blue, gray, and yellow symbols denote Li (Sc), Zn (Ag), and As (C) atoms, respectively, for LiZnAs (ScAgC). Electronic band structures for LiZnAs and ScAgC are shown in (b) and (c), respectively, and the corresponding phonon dispersions are presented in (d) and (e). The Fermi level ($\varepsilon_F$) is set at the top of the valence band.
• Figure 2: Renormalization and temperature dependence of the VBM and CBM states at the $\Gamma$ point obtained from both LQE and OTMS methods for (a) LiZnAs and (b) ScAgC. The blue lines indicate the ZPR, connecting the DFT eigenvalues with the renormalized energy at 0 K.
• Figure 3: Temperature-dependent electron and hole mobilities using methods based on linearized (SERTA, MRTA) and iterative Boltzmann transport equation (IBTE) for (a) LiZnAs and (b) ScAgC.
• Figure 4: The Seebeck coefficients of n-type of (a) LiZnAs and (b) ScAgC hH materials at different temperatures and increasing electron doping concentrations (n$_e$), calculated using the CRTA, SERTA and MRTA approaches.
• Figure 5: Electrical conductivity of n-type of (a) LiZnAs and (b) ScAgC hH materials at different temperatures and increasing electron doping concentrations (n$_e$), calculated using CRTA, SERTA and MRTA.