Rattling-induced effects of Ag atoms and anomalous phonon transport along with thermoelectric performance in silver-based chalcopyrite AgGaX2 (X = Se, Te)

Abstract

A comprehensive elucidation of phonon transport mechanisms is imperative for the rational design of advanced thermoelectric materials exhibiting intrinsically low lattice thermal conductivity. In this work, the thermal and electrical transport behaviors of AgGaX${}_2$ (X = Se, Te) are comprehensively explored through self-consistent phonon calculations, compressive sensing techniques, and the Boltzmann transport equation. The ultralow lattice thermal conductivity (${\kappa }_L$) of AgGaX${}_2$ is attributed to the strong anharmonicity due to the rattling modes of Ag atoms, strong phonon scattering due to the strong coupling between acoustic and low-frequency optical phonon branches, and the unusually high contribution of optical phonons to thermal conductivity. We also find that the ${\kappa }_L$ of AgGaX${}_2$ (X = Se, Te) exhibits an anomalous trend compared to the conventional mass trend, with ${\kappa }_L$ decreasing when the lighter Se atoms replace Te atoms. This anomalous phonon transport behavior is attributed to the weaker Ag–Se bonding strength, lower avoided crossing frequency, and the dominant contribution of optical phonons to ${\kappa }_L$ (exceeding 60%) in AgGaSe${}_2$. The high band degeneracy and strong dispersion near the valence band maximum (VBM) result in a high power factor ($PF$), which, combined with the ultralow ${\kappa }_L$, leads to excellent thermoelectric performance. Accounting for multiple scattering processes, the peak $ZT$ values of p-type AgGaSe${}_2$ and AgGaTe${}_2$ are predicted to attain 2.53 and 2.71 at 700 K, respectively. The inclusion of spin–orbit coupling (SOC) causes the peak $ZT$ values to decrease to 1.99 and 2.14, representing decreases of 22.1% and 21%, respectively. These results indicate that AgGaX${}_2$ is a promising class of high performance thermoelectric materials, and its unique phonon dynamics and electron transport properties make it promising for thermoelectric applications.