Rattling-Induced Ultralow Thermal Conductivity Leading to Exceptional Thermoelectric Performance in AgIn₅S₈

Rattling has emerged as one of the most significant phenomenon for notably reducing the thermal conductivity in complex crystal systems. In this work, using first-principles density functional theory, we found that rattlers can be hosted in simpler crystal systems such as AgIn₅S₈ and CuIn₅S₈. Rattlers Ag and Cu exhibit weak and anisotropic bonding with the neighboring In and S and reside in a very shallow anharmonic potential well. The phonon spectra of these compounds have multiple avoided crossing of optical and acoustic modes, which are a signature of rattling motion. This leads to ultralow thermal conductivity, which is inversely proportional to mass and frequency span of rattling modes. Even though Ag atoms contribute to the valence band states, the rattler modes of Ag do not scatter carriers significantly, leaving the electronic transport virtually unaffected. Moreover, AgIn₅S₈ possesses a combination of heavy and light valence bands resulting in a very high power factor. A combination of favorable thermal and electronic transport results in a very high figure of merit of 2.2 in p-doped AgIn₅S₈ at 1000 K. The proposed idea of having rattlers in simpler systems can be extended to a wider class of materials, which would accelerate the development of thermoelectric modules for waste energy harvesting.