JNCASR Discovers Rare Phonon Particle-to-Wave Heat Transport

Ultralow Thermal Conductivity Mechanism Identified in Tl₂AgI₃

Researchers at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru , have reported a major advance in condensed matter physics by identifying an unconventional mechanism of heat transport in solids. The study, published in Proceedings of the National Academy of Sciences (PNAS) , reveals a particle-to-wave crossover in phonon behaviour within a crystalline material exhibiting local disorder.

Traditionally, heat conduction in solids is described by the phonon gas model , where phonons behave like particles undergoing scattering across a crystal lattice. The new findings demonstrate a distinct regime dominated by wave-like coherence .

Breakdown of Conventional Thermal Theory

The research focused on Tl₂AgI₃ , a zero-dimensional inorganic metal halide characterised by discrete cluster-like structural units rather than an extended lattice. The material displays an exceptionally low lattice thermal conductivity of ~0.18 W/m·K .

Contrary to standard predictions, thermal conductivity becomes nearly temperature-independent above 125 K . Around 175 K , wave-like phonon transport surpasses particle scattering, signalling a breakdown of the classical phonon gas description.

Crystal Chemistry and Anharmonic Effects

The phenomenon is linked to local structural distortions driven by strong cation–cation repulsion , consistent with Pauling’s third rule . These distortions generate extreme lattice anharmonicity , sharply reducing phonon mean free paths.

As scattering intensifies, heat propagation transitions to a regime governed by phonon coherence and tunnelling between localised vibrational states.

Advanced Analytical Framework

Led by Prof. Kanishka Biswas , the team employed synchrotron X-ray pair distribution function analysis, Raman spectroscopy, and low-temperature thermal transport measurements. Theoretical interpretation used the linearised Wigner transport equation , enabling separation of particle-like and wave-like contributions.

Exam-Focused Points

• Tl₂AgI₃ → Zero-dimensional metal halide

• Thermal conductivity → Ultralow (~0.18 W/m·K)

• Phonon gas model → Classical heat transport theory

• Pauling’s third rule → Cation repulsion & stability

• Wigner transport equation → Wave-like heat analysis