Researchers at JNCASR, Bengaluru, have identified an unusual heat-transport mechanism in the inorganic metal halide Tl₂AgI₃. The material shows a crossover from conventional particle-like phonon transport to wave-like coherent transport. The finding reshapes understanding of thermal conduction in crystalline solids with local disorder and may aid the design of next-generation thermoelectrics and thermal management materials.
What Was Discovered
Tl₂AgI₃ displays exceptionally low lattice thermal conductivity of about 0.18 W/m·K. Above roughly 125 K, its thermal conductivity becomes nearly temperature independent. This is unlike normal crystals, where thermal conductivity usually falls with rising temperature. The result indicates a breakdown of the standard phonon gas model.
How Heat Flow Changes
At low temperatures, phonons travel like particles through the crystal lattice. As temperature rises, strong local distortions in silver atoms increase lattice anharmonicity. The phonon mean free path becomes shorter than the atomic spacing. Heat then begins to move through wave-like coherence, with phonons tunnelling between local vibrational states rather than scattering as independent particles. The crossover to dominant wave-like transport occurs around 175 K.
Crystal Chemistry and Mechanism
The material has a zero-dimensional, cluster-like structure rather than a continuous three-dimensional network. The study links the behaviour to cation–cation repulsion, consistent with Pauling’s third rule. Dense coordination units create local instability and strong bond distortion. This promotes phonon localisation and suppresses conventional heat flow. The material therefore behaves like a crystal in structure but like a glass in thermal conduction.
Methods and Significance
The team used synchrotron X-ray pair distribution function analysis, low-temperature thermal transport measurements, Raman spectroscopy, and first-principles calculations. A linearised Wigner transport equation helped separate particle-like and wave-like contributions. The study provides a new strategy for engineering ultralow thermal conductivity through local lattice instability and phonon coherence.
Last Modified: April 28, 2026