New crystal cuts heat flow to boost waste-heat power

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A team at Institute of Science Tokyo says it has found a way to combine two properties that usually work against each other in thermoelectric materials: strong electrical performance and very low thermal conductivity. The material, TlFe1.6Se2, is a layered bulk crystal that periodically embeds atomically thin iron selenide (FeSe) layers while also incorporating ordered iron vacancies.

That combination matters because thermoelectric devices generate electricity from temperature differences, such as waste heat from factories, automobiles, and power plants. To work well, a material must conduct electricity efficiently while blocking heat flow. In practice, getting both at once has been difficult.

The Science Tokyo group, led by Professor Takayoshi Katase of the Materials and Structures Laboratory, reports in the Journal of Materials Chemistry A that TlFe1.6Se2 delivers two advantages at once. The embedded FeSe layers raise the thermoelectric power factor compared with conventional bulk FeSe, mainly through a much larger Seebeck coefficient. According to the researchers, that shows the unusual electronic behavior of atomically thin FeSe can be carried into a practical bulk crystal.

The second gain is thermal. The crystal’s Fe vacancies distort nearby atomic bonds and strongly scatter phonons, which suppresses heat transport. Katase added that the presence of heavy Tl atoms and the material’s complex layered structure further reduce phonon velocity and increase scattering.

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At around 180 °C, the material undergoes a reversible transition from a Fe-vacancy-ordered phase to a Fe-vacancy-disordered phase. In that regime, its thermal conductivity drops to about 0.2 W m-1 K-1, which the researchers say is comparable to, or lower than, state-of-the-art thermoelectric materials.

Electrical transport and the vacancy effect

The ordering of Fe vacancies also changes electrical behavior. In the ordered phase, the Seebeck coefficient exceeds 100 μV K-1, producing a thermoelectric power factor about five times larger than in the disordered phase. The team attributes that jump to electronic-structure changes caused by the ordered arrangement of vacancies.

“This work demonstrates the effectiveness of a new design concept in which the functionality of low-dimensional materials is embedded within bulk crystals. The results provide a promising pathway for the development of next-generation thermoelectric materials that overcome conventional trade-offs between electrical and thermal transport properties.”

The researchers say the same strategy could be extended to related FeSe compounds containing potassium, rubidium, or cesium, which also offer FeSe layers and tunable Fe-vacancy concentrations.

The paper is Xinyi He et al, Simultaneous enhancement of power factor and suppression of thermal conductivity in bulk TlFe 1.6 Se 2 via embedded atomically thin FeSe layers, published in 2026 in the Journal of Materials Chemistry A, DOI: 10.1039/d6ta02075e.