Exceptional thermoelectric performance and strong high-order phonon anharmonicity in the ternary tellurides X AgTe ( X = K , Rb , Cs )

The enhancement of thermoelectric conversion efficiency is exciting but quite challenging due to the conflicting relationships among the key parameters for the thermoelectric performance. Herein, we comprehensively investigate the thermoelectric properties of the novel $X\mathrm{AgTe}$ ($X=\mathrm{K}$, Rb, Cs) materials using advanced first-principles calculations. These materials exhibit favorable stability and relatively low formation energies, making them amenable to preparation under conventional conditions. Their unique hierarchical bonding configuration and the rattling vibration modes of Ag induce the strong high-order phonon anharmonicity. On the other hand, their distinctive electronic structure with the coexistence of high-dispersion valleys and flat band edges leads to a high power factor. As a result, the proposed $X\mathrm{AgTe}$ series possesses both inferior thermal transport and superior electronic transport properties, consequently yielding the outstanding thermoelectric performance. Finally, the impressive peak thermoelectric figure of merit, $ZT>3$, for $p$-type doping, and $ZT>2$ for $n$-type doping were obtained. This study elucidates the underlying physical mechanism for the exceptional thermoelectric performance of $X\mathrm{AgTe}$ and opens pathways for the practical applications of thermoelectric materials.