JNCASR Discovers Wave-Like Heat Transport in Tl₂AgI₃

In a major scientific breakthrough, researchers at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru, have uncovered an unusual mechanism of heat transport in solids that challenges conventional understanding. The study demonstrates a rare particle-to-wave crossover in phonon behaviour within a crystalline material exhibiting local disorder. The findings, published in Proceedings of the National Academy of Sciences (PNAS), have significant implications for next-generation thermoelectrics and thermal management technologies.

Heat in solids is typically transported by phonons behaving as particle-like entities that scatter across crystal lattices. This classical “phonon gas” model has shaped materials science for decades. However, the new research reveals a distinct regime where heat propagates through wave-like coherence.

Breakdown of the Phonon Gas Model

The team studied Tl₂AgI₃, a zero-dimensional inorganic metal halide with discrete cluster-like building blocks rather than an extended three-dimensional network. The material exhibits an exceptionally low lattice thermal conductivity of about 0.18 W/m·K.

Instead of decreasing steadily with rising temperature, as predicted by the phonon gas model, thermal conductivity becomes nearly temperature-independent above 125 K. Around 175 K, wave-like phonon transport overtakes particle-based scattering, signalling a breakdown of conventional theory.

Role of Crystal Chemistry and Anharmonicity

The discovery draws inspiration from Pauling’s third rule, which links atomic arrangement and structural stability. Strong cation–cation repulsion within densely connected coordination units leads to local structural distortions.

These distortions induce extreme lattice anharmonicity, drastically shortening the average phonon mean free path. As phonon scattering intensifies, heat transport transitions to a regime dominated by wave-like coherence and tunnelling between localised vibrational states.

Advanced Experimental and Theoretical Tools

The study was led by Prof. Kanishka Biswas of the New Chemistry Unit, JNCASR. Researchers combined synchrotron X-ray pair distribution function analysis, Raman spectroscopy, low-temperature thermal transport measurements, and advanced first-principles calculations.

They applied a linearised Wigner transport equation developed by Prof. Swapan K. Pati’s group to distinguish particle-like and wave-like contributions. The analysis confirmed a coherence-driven transport regime at elevated temperatures.

Important Facts for Exams

• Tl₂AgI₃ is a zero-dimensional inorganic metal halide with ultralow thermal conductivity (~0.18 W/m·K).
• Phonons usually follow the “phonon gas” model in crystalline solids.
• Pauling’s third rule relates cation–cation repulsion to crystal stability.
• The linearised Wigner transport equation helps analyse wave-like heat transport.

Implications for Thermal Engineering

Researchers describe Tl₂AgI₃ as behaving simultaneously like a crystal and a glass—retaining long-range order while conducting heat in a glass-like manner. The findings propose a new design strategy: engineering local lattice instability to induce phonon localisation and coherence. This approach opens avenues for advanced thermoelectric materials and efficient thermal management systems, reinforcing India’s growing contribution to fundamental materials research.