Unusual Particle-to-Wave Phonon Heat Transport in Solids

Why in News?

Recently, Indian researchers discovered a rare particle-to-wave-like phonon heat transport mechanism in Tl₂AgI₃.

• Tl₂AgI₃ – It is a zero-dimensional inorganic metal halide crystalline material with discrete cluster-like building blocks.
  • i.e., A crystal made of tiny isolated groups of atoms.
• Developed by – Researchers at Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru.
• Nodal Authority – Department of Science and Technology (DST).
• Components – Tl₂AgI₃ is composed of Thallium (Tl), Silver (Ag), and Iodine (I) atoms.
• Discrete cluster of subunits include (Tl₆I)⁵⁺ and (Ag₃I₈)⁵⁻ in the crystal structure.
• Working Principle – Atomic Repulsion and Distortion – Pauling’s third rule–driven cation–cation repulsion causes local structural distortions and anharmonicity.
  • i.e., Positive atoms pushed too close move out of place, creating a shaky structure that blocks heat flow.
• Phonon Trapping – This leads to phonon localization and breakdown of the phonon-gas model.
  • i.e., Heat vibrations get trapped instead of flowing normally through the material.
• Wave-like Heat Tunnelling – With rising temperature, heat transport shifts from particle-like scattering to wave-like coherent tunnelling, analysed using the linearized Wigner transport equation (LWTE).
  • i.e., At higher temperatures, heat moves like waves tunnelling through obstacles rather than flowing smoothly.
• Key Features – Ultralow heat flow – The material blocks heat so effectively that it performs ultralow thermal conductivity.
• Heat-proof stability – Once the material reaches a certain warmth, its ability to block heat stays constant regardless of how much hotter it gets, unlike most materials that change.
• Applications –
  • Thermal insulators.
  • Thermoelectric materials.
  • Advanced thermal management technologies.
• Benefits – Provides a new design strategy to suppress heat transport using structural confinement and local disorder.
• Enhances understanding of phonon physics in low-dimensional solids.
• Significance – Challenges the classical phonon-gas model of heat conduction in crystals.
• Establishes a mixed phononic regime (particle + wave) in crystalline solids.
• Positions India as a leader in fundamental materials research with technological relevance.

Reference

PIB | Unusual Particle-to-Wave Phonon Heat Transport in Solids