Simultaneous enhancement of power factor and suppression of thermal conductivity in bulk TlFe1.6Se2 via embedded atomically thin FeSe layers

Abstract

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 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 around 200 °C. Although V_Fe-disordered phase exhibits high electrical conductivity, carrier compensation suppresses Seebeck coefficient and limits PF. In contrast, V_Fe-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 V_Fe-ordered phase is lower than those of representative thermoelectric chalcogenides, and that of V_Fe-disordered phase further decreases to ~0.2 W/(m·K) at 500 °C due to V_Fe-induced bond heterogeneity. Consequently, the dimensionless figure of merit (ZT) of TlFe_1.6Se₂ reaches ~0.2 at 50 °C in V_Fe-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.