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

Crystalline solids exhibiting inherently low lattice thermal conductivity (κ _L) are of great importance in applications such as thermoelectrics and thermal barrier coatings. However, κ _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 κ _L value of ∼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 ∼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 κ _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 κ _L value. Our results demonstrate that the liquid-like heat transfer could indeed exist in a well-ordered crystal.