Low Sound Velocity Contributing to the High Thermoelectric Performance of Ag₈SnSe₆

TLDR

Conventional thermoelectric design focuses on phonon scattering to reduce lattice thermal conductivity. The study proposes slow phonon propagation as an effective strategy for high‑performance thermoelectrics. Using Ag₈SnSe₆, which has exceptionally low sound velocity, the authors measured lattice thermal conductivity of 0.2 W m⁻¹ K⁻¹ across all temperatures. The low sound velocity in Ag₈SnSe₆ enables Nb‑doped samples to reach peak zT > 1.2 and average zT ≈ 0.8, confirming low sound velocity as a guiding principle for high‑performance thermoelectrics.

Abstract

Conventional strategies for advancing thermoelectrics by minimizing the lattice thermal conductivity focus on phonon scattering for a short mean free path. Here, a design of slow phonon propagation as an effective approach for high‐performance thermoelectrics is shown. Taking Ag 8 SnSe 6 as an example, which shows one of the lowest sound velocities among known thermoelectric semiconductors, the lattice thermal conductivity is found to be as low as 0.2 W m −1 K −1 in the entire temperature range. As a result, a peak thermoelectric figure of merit zT > 1.2 and an average zT as high as ≈0.8 are achieved in Nb‐doped materials, without relying on a high thermoelectric power factor. This work demonstrates not only a guiding principle of low sound velocity for minimal lattice thermal conductivity and therefore high zT , but also argyrodite compounds as promising thermoelectric materials with weak chemical bonds and heavy constituent elements.