A major scientific breakthrough in heat transport in solids has been reported by researchers studying crystalline materials with local disorder, revealing a previously unknown mechanism that could influence next-generation thermoelectrics and thermal management technologies.
Heat in solids is typically carried by phonons, which generally behave like particles that scatter as they move through a crystal lattice. This classical “phonon gas” model has guided materials design for decades.
However, scientists at Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), an autonomous institute of the Department of Science and Technology (DST), have demonstrated a rare particle-to-wave crossover in phonon behavior.
Heat Transport in Solids: Wave-like phonon coherence observed in Tl₂AgI₃
The researchers observed this unusual phenomenon in a zero-dimensional inorganic metal halide, Tl₂AgI₃, where phonons stop behaving like particles and instead propagate through wave-like coherence, tunnelling between localized vibrational states.
The study, led by Kanishka Biswas from the New Chemistry Unit (NCU), was published in Proceedings of the National Academy of Sciences (PNAS). The material exhibits an exceptionally low lattice thermal conductivity of about 0.18 W/m·K.
Instead of decreasing continuously with temperature, the thermal conductivity becomes nearly temperature-independent above around 125 K, signalling a breakdown of the conventional phonon gas model.
At the core of this discovery is the crystal chemistry of Tl₂AgI₃, which consists of discrete cluster-like building blocks rather than an extended three-dimensional network.
Heat Transport in Solids: Anharmonic Chemical Bonding
Nearly a century ago, Nobel laureate Linus Pauling established rules linking atomic arrangement to structural stability in crystals. Inspired by Pauling’s third rule – which states that sharing edges or faces between coordination polyhedra increases cation–cation repulsion – the researchers predicted local lattice instability in the material.
Their experiments revealed pronounced local distortions of silver atoms, leading to strongly anharmonic chemical bonding. This extreme anharmonicity enhances phonon scattering to the point that conventional phonon transport collapses. As a result, heat propagates through coherence-driven wave-like tunnelling between localized vibrational states.
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Heat Transport in Solids: Significance of Findings
Commenting on the significance of the finding, Prof. Kanishka Biswas said, “Tl₂AgI₃ 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.”
The team combined synchrotron X-ray pair distribution function measurements, low-temperature thermal transport experiments, Raman spectroscopy, and first-principles theoretical calculations to build a comprehensive understanding of the phenomenon.
They also applied a recently developed linearized Wigner transport equation created by the research group of Swapan K. Pati at JNCASR to distinguish between particle-like and wave-like heat transport. Their analysis showed that wave-like coherence-driven transport overtakes particle-based transport around 175 K.
“This is a rare experimental realization of a concept that was largely theoretical,” Prof. Biswas added. “We show that crystalline solids do not have to be strictly particle-like phonon scattering in how they carry heat. Instead, they can access a mixed regime where wave-like coherence dominates, leading to ultralow and glassy thermal conductivity.”
Heat Transport in Solids: Experimental Work Led by First Author Riddhimoy Pathak
The experimental work was led by first author Riddhimoy Pathak, a Ph.D. student of Prof. Biswas, who carried out synthesis, structural characterization, and thermal transport measurements.
The study’s joint first author, Sayan Paul from the Theoretical Sciences Unit (TSU), contributed theoretical insights into phonon coherence and wave-like heat transport.
The findings establish a new materials-design strategy – using chemical rules and local lattice instability to engineer phonon localization and coherence in crystalline solids – with potential implications for advanced thermal management technologies.
The research benefited from national supercomputing resources and international synchrotron facilities through the India@DESY programme, highlighting India’s growing leadership in fundamental materials science research.
Publication link: https://doi.org/10.1073/pnas.2521353123
