Strong low-energy rattling modes enabled liquid-like ultralow thermal conductivity in a well-ordered solid

PF Liu and XY Li and JY Li and JB Zhu and Z Tong and M Kofu and M Nirei and JP Xu and W Yin and FW Wang and TJ Liang and L Xie and YS Zhang and DJ Singh and J Ma and H Lin and JR Zhang and JQ He and BT Wang, NATIONAL SCIENCE REVIEW, 11 (2024).

DOI: 10.1093/nsr/nwae216

Crystalline solids exhibiting inherently low lattice thermal conductivity (kappa L) are of great importance in applications such as thermoelectrics and thermal barrier coatings. However, kappa L cannot be arbitrarily low and is limited by the minimum thermal conductivity related to phonon dispersions. In this work, we report the liquid-like thermal transport in a well-ordered crystalline CsAg5Te3, which exhibits an extremely low kappa L value of similar to 0.18 Wm-1K-1. On the basis of first-principles calculations and inelastic neutron scattering measurements, we find that there are lots of low-lying optical phonon modes at similar to 3.1 meV hosting the avoided-crossing behavior with acoustic phonons. These strongly localized modes are accompanied by weakly bound rattling Ag atoms with thermally induced large amplitudes of vibrations. Using the two-channel model, we demonstrate that coupling of the particle-like phonon modes and the heat-carrying wave-like phonons is essential for understanding the low kappa L, which is heavily deviated from the 1/T temperature dependence of the standard Peierls theory. In addition, our analysis indicates that the soft structural framework with liquid-like motions of the fluctuating Ag atoms is the underlying cause that leads to the suppression of the heat conduction in CsAg5Te3. These factors synergistically account for the ultralow kappa L value. Our results demonstrate that the liquid-like heat transfer could indeed exist in a well-ordered crystal. Liquid-like heat transfer is observed and explained in a well-ordered crystal, which hosts soft structural framework with liquid-like motions of the fluctuating/rattling Ag atoms.

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