In a major scientific breakthrough, researchers have discovered an unusual mechanism of heat transport in solids that fundamentally reshapes the understanding of how heat flows in crystalline materials. This can have implications in next-generation thermo-electrics and thermal management technologies that have applications in the strategic and industrial domains.
Heat in solids is typically carried by quasi-particles called phonons, which generally behave like particles that scatter as they move through a crystal lattice. This understanding has guided materials design for decades.
Researchers at Jawaharlal Nehru Centre for Advanced Scientific Research have now demonstrated a rare transition in which phonons stop behaving like particles and instead propagate through wave-like activity. This particle-to-wave-like crossover was observed in a newly studied inorganic material based on thalium and silver-iodide, according to information shared by the Ministry of Science and Technology on Friday.
At the heart of this discovery is the unique crystal chemistry of the material, termed as Tl2AgI3. “It 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 localisation and wave-like coherence,” Prof Kanishka Biswas, who undertook the work, said.
“This is a rare experimental realization of a concept that was largely theoretical. We have shown 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,” Prof Biswas added.
The research, published in Proceedings of National Academy of Sciences, a US-based peer-reviewed journal, showed that the material exhibits an exceptionally low lattice thermal conductivity. Remarkably, instead of decreasing continuously with temperature as expected for normal crystals, the thermal conductivity becomes nearly temperature independent above around 125K, signalling a breakdown of conventional phonon gas model.
By combining state-of-the-art synchrotron X-ray pair distribution function measurements, low-temperature thermal transport experiments, Raman spectroscopy, and advanced first-principles theoretical calculation, the researchers provided a comprehensive picture of this phenomenon.
