Computational insights to electron-phonon and phonon-phonon interactions in AgX (X = Br, Cl): Refining thermoelectric property predictions

Abstract

The constant relaxation time (CRT) approach for assessing electronic transport in semiconductors inadequately accounts for the carrier scattering effects. This study investigates electron-phonon (el-ph) and phonon-phonon (ph-ph) interactions in AgX (X = Br, Cl) using GW-approximated Kohn-Sham eigenstates, offering refined insights into the theoretically predicted thermoelectric properties by addressing disparities in electron and hole scatterings. The results reveal prevalence of weak el-ph coupling, dominated by small-angle backscattering of charge carriers, with limited forward scatterings driven by a few highly interacting longitudinal optical (LO) phonons associated with wavevectors near the Brillouin zone center. The materials demonstrate a significant difference in hole and electron scattering rates—approximately 12:1 in AgBr and 9:1 in AgCl between 300 and 600 K—attributable to substantial variance in the density of states at the band edges. Additionally, the analysis of three-phonon interactions establishes a notable association of high-frequency longitudinal acoustic and optical phonons with high scattering rates, which contribute to the ultralow lattice thermal conductivities of these materials. The refined calculations predict maximum zT values for the p-type variants, reaching 0.91 (3.44) in AgBr and 1.71 (4.32) in AgCl at 300 K (600 K). These findings showcase the limitations of the CRT approximation in describing scattering processes in AgX (X = Br, Cl) and highlight the true potential of these compounds for thermoelectric applications.