New Layered Crystal Enables High-Efficiency Thermoelectric Energy Conversion

Thermoelectric materials are prized for their ability to harvest waste heat and turn it into electricity, yet they face a stubborn trade-off: boosting electrical transport typically raises thermal conductivity. A new study from the Institute of Science Tokyo tackles this dilemma by redesigning the internal architecture of a bulk crystal rather than only tuning its composition.

The researchers report TlFe1.6Se2, a layered material in which atomically thin FeSe sheets are periodically embedded inside a bulk host. The concept is to inherit the superior thermoelectric power factor associated with ultrathin FeSe while simultaneously suppressing heat flow in the surrounding crystal.

In their approach, the embedded FeSe layers coexist with ordered iron (Fe) vacancies. These vacancies act as built-in “phonon scatterers.” By disrupting local bonding and creating a complex lattice landscape, the vacancies strongly reduce the mobility of heat-carrying vibrations, lowering lattice thermal conductivity.

Electrical performance improves at the same time. The team finds that Seebeck coefficient values exceed 100 μV K−1 in the Fe-vacancy-ordered phase, delivering a thermoelectric power factor roughly five times larger than in the vacancy-disordered phase. The enhancement is linked to electronic structure changes induced by the vacancy ordering.

A key feature is a reversible temperature-driven transition near 180 °C, where the vacancy arrangement shifts from ordered to disordered. This dynamic behavior further strengthens phonon scattering and pushes thermal conductivity down to about 0.2 W m−1 K−1—at the level of, or lower than, leading thermoelectrics.

The results highlight a “low-dimensional functionality in bulk form” strategy: instead of building devices from films alone, the material embeds the benefits of two-dimensional physics inside a practical bulk crystal. Heavy thallium (Tl) atoms and the complex layered stacking additionally contribute by reducing phonon velocities and increasing scattering.

The publication also suggests broader applicability. Related alkali-intercalated FeSe systems containing potassium, rubidium, or cesium may offer tunable vacancy concentrations, providing a pathway to further optimize thermoelectric performance.

Overall, TlFe1.6Se2 demonstrates that power factor and thermal suppression can be engineered together through structural design—offering a viral, concept-forward blueprint for next-generation waste-heat converters.