Glass-like thermal conductivity and phonon transport mechanism in disordered crystals

Solid materials with ultra-low thermal conductivity (kappa) are of great interest in thermoelectrics for energy conversion or as thermal barrier coatings for thermal insulation. Many low-kappa materials exhibit unique properties, such as weak or even insignificant dependence on temperature (T) for kappa, i.e., an anomalous glass-like behavior. However, a comprehensive theoretical model elucidating the microscopic phonon mechanism responsible for the glass-like kappa-T relationship is still absent. Herein, we take rare-earth tantalates (RE3TaO7) as examples to reexamine phonon thermal transport in defective crystals. By combining experimental studies and atomistic simulations up to 1800 K, we revealed that diffusion-like phonons related to inhomogeneous interatomic bonding contribute more than 70% to the total kappa, overturning the conventional understanding that low-frequency phonons dominate heat transport. Furthermore, due to the bridging effects of interatomic bonding, the kappa of high-entropy tantalates is not necessarily lower than that of medium-entropy materials, suggesting that attempts to reduce kappa through high-entropy engineering are limited, at least in defective fluorite tantalates. The new physical mechanism of multimodal phonon thermal transport in defective structures demonstrated in this work provides a reference for the analysis of phonon transport and offers a new strategy to develop and design low-kappa materials by regulating the inhomogeneity of interatomic bonding. Glass-like thermal conductivity (kappa) in disordered crystals was characterized successfully by multimodal phonon transport. Reducing kappa via high-entropy engineering may be limited due to the differences in interatomic bonding.