Particle-to-Wave heat transport discovered in crystalline solids.
Photo Credit: PIB
New Delhi, February 13, 2026: In a major scientific breakthrough, researchers have uncovered an unusual mechanism of heat transport in solids that fundamentally reshapes our understanding of how heat flows in crystalline materials with local disorder.
The discovery holds significant promise for next-generation thermoelectric and advanced thermal management technologies.
Heat in solids is typically carried by phonons—quantized lattice vibrations—that behave like particles scattering through a crystal lattice.
This classical “phonon gas” model has guided materials research and thermal design for decades.
Scientists discovered heat transport at the JNCASR:
Scientists at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), an autonomous institute of the Department of Science and Technology (DST), have now demonstrated a rare transition in which phonons cease to behave like particles and instead propagate via wave-like coherence, tunneling between localized vibrational states.
This particle-to-wave crossover was observed in a newly studied zero-dimensional inorganic metal halide, TI2AgI3.
The study, led by Kanishka Biswas, Professor at the New Chemistry Unit (NCU), JNCASR, has been published in the prestigious journal Proceedings of the National Academy of Sciences (PNAS).
The material exhibits an exceptionally low lattice thermal conductivity of ~0.18 W/m.K. Remarkably, instead of decreasing continuously with temperature—as expected for normal crystals—the thermal conductivity becomes nearly temperature—independent above ~125K, signaling a breakdown of the conventional phonon model.
At the heart of this discovery lies the unique crystal chemistry of TI2AgI3. Its structure consists of discrete, cluster-like units rather than an extended three—dimensional network.
Nearly a century ago, Nobel laureate Linus Pauling established foundational rules of crystal chemistry, including the principle that sharing edges or faces between coordination polyhedra enhances cation—cation repulsion. Inspired by this idea, the researchers anticipated that strong local cation repulsion could destabilize the lattice.
Guided by this insight, the team experimentally revealed pronounced local distortions of silver atoms, leading to extreme anharmonicity in chemical bonding.
This dramatically increases particle-like phonon scattering, eventually collapsing conventional phonon transport.
As a result, heat begins to flow through wave-like coherence, with phonons tunneling between localized vibrational states rather than travelling as well-defined particles.
Expert’s opinion:
Commenting on the significance of the findings, Prof. Biswas said, “TI2AgI3 is a rare example of a material that behaves simultaneously like a crystal and a glass. It retains long-range crystalline order, yet conducts heat in a glass-like manner due to phonon localization and wave-like coherence.
To arrive at this comprehensive picture, the team combined synchrotron X-ray pair distribution function measurements, low-temperature thermal transport experiments, Raman spectroscopy, and advanced first-principles theoretical calculations.
Crucially, they employed the linearized Wigner transport equation, developed by the group of Swapan K. Patil at JNCASR, to distinguish between particle-like and wave-like heat transport.
Their analysis reveals that coherence-driven wave transport overtakes particle-based transport around 175K.
“This is a rare experimental realization of a concept that was largely theoretical,” Prof. Biswas added. “Crystalline solids do not have to transport heat solely through particle-like phonon scattering.
They can instead access a mixed regime where wave-like coherence dominates, resulting in ultralow, glassy thermal conductivity.
The experimental work was led by the first author, Dr. Riddhimoy Pathak, a Ph.D. student under Prof Biswas, who carried out material synthesis, structural characterization, and thermal transport measurements.
The study also features joint first author Sayan Paul, from the Theoretical Sciences Unit (TSU), JNCASR, who provided key theoretical insight into phonon coherence and wave-like heat flow.
The findings establish a new materials design strategy—using chemical rules and local lattice instabilities to engineer phonon localization and coherence in crystalline solids.
The research benefited from national supercomputing facilities and international synchrotron resources accessed through the India@DESY program.
Overall, this achievement highlights India’s growing leadership in fundamental materials research, demonstrating how deep chemical intuition, combined with advanced experimental and theoretical tools, can uncover entirely new physical regimes with strong technological relevance.
EOM.
