Bonding Hierarchy and Phonon Coherence Enhanced Ultralow Lattice Thermal Conductivity and Excellent Thermoelectric Properties in Cs₂TeI₆

Abstract Lead-free halide perovskites provide a promising solution for efficient thermoelectric materials due to their ultralow lattice thermal conductivity ( κ L ). However, disadvantages such as the electrically resistive nature strongly affect their power factor. In this work, we introduced the Te-based halide perovskites thermoelectric material Cs 2 TeI 6 , which is already known as a promising candidate for photovoltaic applications due to its moderate band gap. Our findings reveal that Cs 2 TeI 6 has an exceptionally ultralow κ L at room temperature, reaching as low as 0.17 W⋅m −1 ⋅K −1 . We found that the four-phonon scattering processes play a dominant role in suppressing the thermal transport, leading to an approximate 50% reduction in its particle-like thermal conductivity κ p at 300 K. The ultralow κ L can be mainly attributed to the strong discrepancy in bonding strength, which induces large anharmonicity. The flat and dense phonon dispersions result in a strong phonon scattering rate, making it easy to generate wavelike phonon tunneling. After accounting for the wavelike thermal conductivity κ c , a nonstandard T −0.30 temperature dependence was observed. Benefiting from the ultralow κ L , n-type Cs 2 TeI 6 is predicted to achieve an extraordinary ZT of 2.26 at 700 K. This work highlights a pathway for searching high-performance and low-cost thermoelectrics based on lead-free halide perovskites.