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

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid State Chemistry

Background:

• Low lattice thermal conductivity (κL) is crucial for thermoelectric and thermal barrier applications.
• Phonon dispersions typically limit how low κL can become in crystalline solids.
• Existing theories, like Peierls theory, describe heat transport but may not capture all phenomena.

Purpose of the Study:

• To investigate the mechanism behind extremely low lattice thermal conductivity in crystalline CsAg5Te3.
• To explore the possibility of liquid-like thermal transport in ordered crystalline materials.
• To understand the role of specific phonon modes and atomic vibrations in heat conduction.

Main Methods:

• Experimental synthesis and characterization of crystalline CsAg5Te3.
• First-principles calculations to model phonon behavior and interactions.
• Inelastic neutron scattering measurements to probe phonon dispersions and dynamics.
• Application of the two-channel model to analyze heat transport contributions.

Main Results:

• Achieved an exceptionally low lattice thermal conductivity (κL) of ~0.18 Wm⁻¹K⁻¹ in CsAg5Te3.
• Identified low-lying optical phonon modes (~3.1 meV) exhibiting avoided-crossing with acoustic phonons.
• Observed weakly bound silver (Ag) atoms with large, thermally induced vibrational amplitudes.
• Demonstrated that coupling between localized, particle-like phonon modes and wave-like phonons governs heat transport.

Conclusions:

• The study reveals liquid-like heat transfer in a well-ordered crystalline solid.
• Soft structural framework and rattling Ag atoms contribute to suppressed heat conduction.
• The observed behavior deviates significantly from the standard 1/T temperature dependence predicted by Peierls theory.