Simultaneous enhancement of power factor and suppression of thermal conductivity in bulk TlFe1.6Se2via embedded atomically thin FeSe layers

Abstract

FeSe in the monolayer limit exhibits extremely large thermoelectric power factors (PFs). Extending the high-PF concept from two-dimensional FeSe to bulk materials, together with the suppression of lattice thermal conductivity, enables higher-performance thermoelectrics. Here, layered TlFe_1.6Se₂ 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 (V_Fe)-ordered to-disordered states at around 200 °C. Although the V_Fe-disordered phase exhibits high electrical conductivity, carrier compensation suppresses the Seebeck coefficient and limits PF. In contrast, the V_Fe-ordered phase shows an enhanced Seebeck coefficient associated with Mott gap formation, resulting in improved PF which is much higher than that of bulk FeSe. The lattice thermal conductivity of the V_Fe-ordered phase is lower than that of representative thermoelectric chalcogenides and that of the V_Fe-disordered phase further decreases to ∼0.2 W (m⁻¹ K⁻¹) at 500 °C due to the V_Fe-induced bond heterogeneity. Consequently, the dimensionless figure of merit (ZT) of TlFe_1.6Se₂ reaches ∼0.2 at 50 °C in the V_Fe-ordered phase, which is two orders of magnitude higher than that of 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.