Developing thermoelectric materials that efficiently convert waste heat into electricity remains challenging because high electrical performance and low thermal conductivity are difficult to achieve simultaneously. Researchers at Science Tokyo developed a layered crystal, TlFe1.6Se2, that embeds atomically thin iron selenide (FeSe) layers within a bulk material. The crystal combines a high thermoelectric power factor with exceptionally low thermal conductivity, demonstrating a promising strategy for designing next-generation materials for waste heat energy recovery.
Thermoelectric technology, which converts waste heat from factories, automobiles, and power plants into electricity, is expected to play an important role in building a carbon-neutral society. In thermoelectric power generation, electricity is produced using a temperature difference across a material. To achieve high power-generation performance, materials must efficiently convert heat into electrical power while maintaining the temperature difference that drives power generation. However, these two requirements are generally difficult to satisfy simultaneously. Establishing new material-design strategies that combine high thermoelectric performance with low thermal conductivity has therefore been a major challenge.
Superconductors have rarely been considered as thermoelectric materials because they generally exhibit poor thermoelectric performance. However, recent studies have revealed that atomically thin iron selenide (FeSe), an iron-based superconductor, exhibits an exceptionally high thermoelectric power factor. This remarkable performance, however, has only been demonstrated in ultrathin films, making practical thermoelectric applications difficult. Moreover, bulk FeSe has a relatively high thermal conductivity, limiting its thermoelectric performance.
To overcome these limitations, a research team led by Professor Takayoshi Katase from the Materials and Structures Laboratory, Institute of Science Tokyo (Science Tokyo), Japan, developed a layered crystal with thallium (Tl), TlFe1.6Se2, in which atomically thin FeSe layers are periodically embedded within a bulk crystal together with ordered Fe vacancies. This unique crystal design was conceived to combine the high thermoelectric power factor of atomically thin FeSe with the low thermal conductivity introduced by ordered Fe vacancies.
In a study made available online on April 30, 2026, and published in Volume 14, Issue 37 of the Journal of Materials Chemistry A on June 23, 2026, the team demonstrated two key advantages of TlFe1.6Se2.
First, the embedded FeSe atomic layers produce a much higher thermoelectric power factor than conventional bulk FeSe. This enhancement is primarily attributed to a significantly larger Seebeck coefficient, demonstrating that the unique electronic properties of atomically thin FeSe can be successfully incorporated into a bulk crystal.
Second, TlFe1.6Se2 exhibits exceptionally low thermal conductivity. The crystal naturally contains Fe vacancies within the FeSe layers. These Fe vacancies distort the surrounding atomic bonds and strongly scatter heat-carrying phonons, greatly reducing heat transport. "In addition, the incorporation of heavy Tl atoms and the resulting complex layered structure contribute to phonons' reduced velocities and enhanced scattering," adds Katase.
Furthermore, at around 180 °C, the material undergoes a reversible transition from a Fe-vacancy-ordered phase to a Fe-vacancy-disordered phase, which further enhances phonon scattering and reduces thermal conductivity to approximately 0.2 W m-1 K-1, comparable to or even lower than that of state-of-the-art thermoelectric materials.
The Fe-vacancy ordering also strongly influences electrical transport. In the Fe-vacancy-ordered phase, the Seebeck coefficient exceeds 100 μV K-1, resulting in a thermoelectric power factor approximately five times larger than that of the disordered phase. The researchers attribute this enhancement to changes in the electronic structure associated with the ordered arrangement of Fe vacancies.
These results demonstrate a new material-design strategy that combines the high thermoelectric power factor of atomically thin materials with the exceptionally low thermal conductivity introduced by ordered Fe vacancies. Rather than simply optimizing existing thermoelectric compounds, this approach embeds the functionality of atomically thin materials into practical bulk crystals, opening a new direction for thermoelectric materials design.
"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," says Katase.
The researchers believe that this approach could be extended to related FeSe compounds containing potassium, rubidium, or cesium. These materials also contain FeSe layers and tunable Fe-vacancy concentrations, making them promising platforms for optimizing thermoelectric performance.
