Rattling-Induced Ultralow Lattice Thermal Conductivity in Simple Systems

Rattling motion is one of the significant phenomena for achieving notable reduction in the thermal conductivity in complex crystal systems. To carry forward the advantages of rattling to simple crystal systems, we explored it in simpler cubic compounds AgIn₅S₈ and CuIn₅S₈. The weak and anisotropic bonding of Ag and Cu with the neighboring In and S causes the rattling motion, which result in very shallow anharmonic potential well for the rattlers Ag and Cu. 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. 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. A combination of favorable thermal and electronic transport results in unprecedented 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.

Reference: ACS Appl. Mater. Interfaces, 11, 33894−33900, 2019