The New Cubic Argyrodites: Ag ₄ M _0.5 S ₂ Te (M = Sn and Ge) with Ultralow Thermal Conductivity

Abstract The Ag‐based superionic argyrodites with liquid‐like ultralow lattice thermal conductivity have garnered widespread attention as promising thermoelectric materials. However, the inferior performance of n‐type argyrodites does not match the p‐type counterpart for device construction. Here, the two new cubic n‐type Ag‐based argyrodite compounds Ag 4 M 0.5 S 2 Te (M = Sn and Ge) are synthesized. They crystallize in the space group with excellent structural stability. The unique triangular [AgS 2 Te] 5− , rod‐like [AgSTe] 3− , and complex cage‐like [Te 4 Ag 36 ] 28+ clusters together with the vibrations of weakly bonded Ag + ions result in large lattice anharmonicity and enhance the scatter of acoustic phonons. These structure characters lead to the ultralow lattice thermal conductivity ( κ lat ) of 0.30–0.32 W m −1 K −1 near the amorphous limit 0.24 W m −1 K −1 at 525–823 K. Moreover, the power factor enhanced from 1.53 µW cm −1 K −2 for Ag 4 Sn 0.5 S 2 Te to 2.61 µW cm −1 K −2 by introducing Te deficiencies, resulting in a high ZT of 0.74 at 823 K for Ag 4 Sn 0.5 S 2 Te 0.92 , which is 95 % higher than that of the pristine sample.