Raising a glass

For 15 years or so, Bengaluru-based solid-state chemist Kanishka Biswas has been studying how heat is transported in crystalline materials. The application of heat transfer has immense potential. While ultra-high thermal conductive crystalline materials can be applied in heat transmission and dissipation, extremely low thermal conductive materials can be used for thermoelectric applications, where waste heat is captured and gainfully used, and in thermal barrier coatings.

Biswas and his team at the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) have now made a fundamental discovery that could lead to materials with ultra-low thermal conductivity. Their work, which appeared in the Proceedings of the National Academy of Sciences in January (bit.ly/thermoelectric-Biswas), challenges the long-standing belief that only glasses or disordered solids had glass-like low thermal conductivity. They achieved this by creating anharmonic vibrations in a material otherwise structurally ordered.

In crystalline solids, heat travels as vibrations of atoms locked in a repeating lattice. These vibrations are quantised into packets called phonons. For decades, scientists believed phonons behaved like particles: they bounced, scattered, and ricocheted. The more they scattered, the less efficiently heat flowed.

In glasses, where atoms lack long-range order, this scattering is intense. Thermal conductivity drops dramatically. That's why glass wool is good for insulation. Crystals, on the other hand, are orderly. Their symmetry typically allows phonons to travel more freely, making them better heat conductors.

But when a structural disorder is introduced in a low thermal conductive crystal, thermal conductivity drops further. This is because anharmonicity in lattice vibrations due to the structural disorder leads to what scientists call phonon localisation. "When the mean free path of phonons becomes shorter than the interatomic spacing, phonons localise. Heat transport then shifts from particle-like diffusion to wave-like propagation," Biswas explains.

In most crystals both these regimes — particles and waves — co-exist. What makes this discovery fundamental is the clear identification of a crossover regime driven purely by intrinsic vibrational properties and not external disorder.

For the study, the scientists used a zero-dimensional inorganic metal halide — dithallium silver triiodide (Tl₂AgI₃) — a crystal consisting of discrete cluster-like units of alternating thallium and silver-iodine polyhedral subunits.

To induce lattice instability, the team invoked American chemist Linus Pauling's third empirical rule — formulated nearly a century ago — which states that the stability of an ionic structure decreases when polyhedra share edges or faces. Such sharing brings positively charged ions closer together, increasing electrostatic repulsion.

By designing the crystal to follow this rule, the researchers enhanced local lattice instability and strong anharmonic vibrations. "In doing so, we made a crystal behave more like a glass," says Biswas. The crossover phenomenon emerges from a chemically engineered lattice instability — an interface of chemistry and physics.

An ultra-low thermal conducting crystal has very efficient thermoelectric properties. From the exhaust pipe of a car to the blazing interior of a thermal power plant, vast amounts of energy escape every day as waste heat. Globally, more than half of the energy generated dissipates into the surroundings. Capturing even a fraction of that and converting it into electricity would change the economics of energy.

Angshuman Nag, Associate Professor of Chemistry at the Indian Institute of Science Education and Research (IISER) Pune, sees this as "exciting" work. The particle-to-wave phonon crossover fundamentally changes the way vibrational energy propagates in solids. This crossover suppresses thermal transport because of intrinsic vibrational characteristics, rather than extrinsic scattering alone," says Nag, who is not connected with the study.

This behaviour, he says, is relevant for thermoelectric energy conversion, where reducing thermal conductivity without significantly degrading electronic transport is essential for achieving high conversion efficiency. "Such thermoelectric materials are promising candidates for converting industrial waste heat to useful electrical energy. Besides, such crossover is expected to influence electron-phonon coupling, tailoring optoelectronic properties of different materials," says Nag, who specialises in developing novel semiconductor nanocrystals for optoelectronic applications.

Biswas, however, says they are still to explore practical applications of their discovery. But if it is replicated across other materials systems, the approach could open an entirely new design paradigm for thermoelectric and thermal barrier materials.