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

Lead-free halide perovskites provide a promising solution for efficient thermoelectric materials due to their ultralow lattice thermal conductivity (\kappa_\rm 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_2TeI_6, which is already known as a promising candidate for photovoltaic applications due to its moderate band gap. Our findings reveal that Cs_2TeI_6 has an exceptionally ultralow \kappa_\rm L at room temperature, reaching as low as 0.17 W\cdotm^-1\cdotK^-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 \kappa_\rm p at 300 K. The ultralow \kappa_\rm 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 \kappa_\rm c, a nonstandard T^-0.30 temperature dependence was observed. Benefiting from the ultralow \kappa_\rm L, n-type Cs_2TeI_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.