Fluctuating bonding leads to glass-like thermal conductivity in perovskite rare-earth tantalates

Understanding the glass-like thermal conductivity (κ) in simple-composition crystalline materials is critical for the development and design of low‒κ materials. Here, we report the origin of glass-like low κ in binary crystalline tantalates RETa3O9 (RE = Nd, Sm, Eu, Gd) by atomistic simulation. It is found that the low κ is mainly due to the rattling effect of RE3+ cations arising from the large difference in the interatomic bonding between O‒RE and O‒Ta. The vibrational mode decomposition results reveal that most modes in RETa3O9 fall in the Ioffe–Regel regime and exhibit a strongly diffusive nature due to the fluctuating interatomic force of O‒Ta in the TaO6 octahedron. The dual-phonon model reveals that the vast majority (>97%) of heat in RETa3O9 is transported by diffusive modes rather than propagating modes. Consequently, the thermal conduction in RETa3O9 exhibits a unique glass-like nature. Contrary to conventional belief, the optical phonon modes in RETa3O9 play a significant role in thermal conduction. Overall, the new insight into the link between chemical binding and glass-like κ serves to the development and design of low‒κ materials for thermoelectric and thermal management applications.