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 RETa ₃O ₉ (RE = Nd, Sm, Eu, Gd) by atomistic simulation. It is found that the low κ is mainly due to the rattling effect of RE ³⁺ 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 RETa ₃O ₉ fall in the Ioffe–Regel regime and exhibit a strongly diffusive nature due to the fluctuating interatomic force of O‒Ta in the TaO ₆ octahedron. The dual-phonon model reveals that the vast majority (>97%) of heat in RETa ₃O ₉ is transported by diffusive modes rather than propagating modes. Consequently, the thermal conduction in RETa ₃O ₉ exhibits a unique glass-like nature. Contrary to conventional belief, the optical phonon modes in RETa ₃O ₉ 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.