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FeSe in the monolayer limit exhibits extremely large thermoelectric power factors (PF). Extending the high-PF concept from two-dimensional FeSe to bulk materials, together with lattice thermal conductivity suppression, enables higher-performance thermoelectrics. Here, layered TlFe1.6Se2 is identified as a model system consisting of atomically thin two-dimensional FeSe layers separated by Tl atoms; i.e., FeSe monolayers are naturally confined within a bulk crystal. This compound uniquely exhibits a transition from Fe-vacancy (VFe)-ordered to -disordered states around 200 °C. Although VFe-disordered phase exhibits high electrical conductivity, carrier compensation suppresses Seebeck coefficient and limits PF. In contrast, VFe-ordered phase shows an enhanced Seebeck coefficient associated with Mott gap formation, resulting in improved PF much higher than that of bulk FeSe. The lattice thermal conductivity of VFe-ordered phase is lower than those of representative thermoelectric chalcogenides, and that of VFe-disordered phase further decreases to ~0.2 W/(m·K) at 500 °C due to VFe-induced bond heterogeneity. Consequently, the dimensionless figure of merit (ZT) of TlFe1.6Se2 reaches ~0.2 at 50 °C in VFe-ordered phase, which is two orders of magnitude higher than bulk FeSe. These results demonstrate that confining FeSe monolayers within a bulk crystal, alongside vacancy order–disorder control, is an effective design strategy for next-generation thermoelectrics.

Rights:

• Creative Commons BY Attribution 4.0 International
  

Keyword: Thermoelectricity, Thermal conductivity

Date published: 2026-04-30

Publisher: Royal Society of Chemistry (RSC)

Journal:

• Journal of Materials Chemistry A (ISSN: 20507488) vol. 14 issue. 37 p. 24666-24677

Funding:

• Japan Society for the Promotion of Science 20H00302
• Japan Society for the Promotion of Science 21H04612
• Japan Society for the Promotion of Science 22H04964
• Japan Society for the Promotion of Science 24H00314
• Japan Society for the Promotion of Science 24H00376
• Japan Society for the Promotion of Science 24K21671
• Japan Society for the Promotion of Science 25K23539
• Japan Society for the Promotion of Science 26K01206
• Kanagawa Institute of Industrial Science and Technology
• Ministry of Education, Culture, Sports, Science and Technology JPMXP1122683430
• Tokyo Institute of Technology

Manuscript type: Author's version (Accepted manuscript)

MDR DOI: https://doi.org/10.48505/nims.6393

First published URL: https://doi.org/10.1039/d6ta02075e

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Updated at: 2026-07-10 08:51:05 +0900

Published on MDR: 2026-07-10 10:24:19 +0900