JNCASR team finds wave-like heat transport in TlCu5Se3, boosting thermoelectric potential

Researchers at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Bengaluru, have identified an unusual wave-like heat transport regime in a copper chalcogenide material, thallium copper selenide (TlCu5Se3), a discovery that could support more efficient thermoelectric conversion of waste heat into electricity.

In crystalline solids, heat is conventionally carried by particle-like phonons whose mean free path is far larger than the spacing between atoms. In glasses, strong disorder and anharmonicity — where atomic vibrations become asymmetric and depart from normal behaviour — shrink that path to nearly the interatomic scale, suppressing conventional phonon propagation. The new work points to an intermediate regime in which long-range crystallographic order coexists with highly localised, closely spaced vibrational modes that interact strongly, allowing thermal energy to move through wave-like coherence between modes as well as through particle-like phonon propagation.

Conventional superionic materials can attain low lattice thermal conductivity through highly mobile ions, but excessive ion migration can destabilise the structure and hurt thermoelectric performance. Confined ion diffusion — ionic motion restricted within a complex crystalline framework — offers a way to suppress heat transport while preserving stability.

The JNCASR team, led by Prof. Kanishka Biswas with Ph.D. student Sayantoni Choudhury and Dr. Animesh Bhui of the New Chemistry Unit, examined the structural and thermoelectric properties of TlCu5Se3 through experiments and advanced theoretical calculations. The compound crystallises in a tetragonal structure with a three-dimensional cloverleaf knot-like framework containing open channels along the crystallographic c-axis. Its bonding hierarchy was expected to confine the motion of copper atoms rather than permit long-range migration.

Molecular-dynamics simulations, carried out with Prof. Umesh V. Waghmare and Dr. Prasad V. Matukumilli of the Theoretical Sciences Unit at JNCASR, showed that copper atoms exhibit localised dynamic disorder instead of the liquid-like long-range diffusion seen in superionic copper chalcogenides. This confined motion acts mainly as a source of strong lattice anharmonicity, causing heat to propagate through wave-like coherence, with phonons tunnelling between localised vibrational states rather than moving as well-defined particles. To capture this behaviour, the researchers went beyond the conventional phonon-gas model and used a unified formalism of thermal transport that accounts for both particle-like propagation and wave-like coherence between phonon branches.

The combination of this unusual thermal transport with favourable electronic transport produced a thermoelectric figure of merit (zT) of 1.7 — among the highest reported for pristine ternary chalcogenides. The findings, published in Science Advances, are relevant to thermal barrier coatings, thermoelectric energy conversion and thermal decoherence-free quantum technology, and suggest possible applications in power plants, cement and steel plants, automobiles, data centres and battery heat management systems.