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

ABSTRACT

Solid materials with ultra-low thermal conductivity (κ) are of great interest in thermoelectrics for energy conversion or as thermal barrier coatings for thermal insulation. Many low-κ materials exhibit unique properties, such as weak or even insignificant dependence on temperature (T) for κ, i.e., an anomalous glass-like behavior. However, a comprehensive theoretical model elucidating the microscopic phonon mechanism responsible for the glass-like κ-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 κ, overturning the conventional understanding that low-frequency phonons dominate heat transport. Furthermore, due to the bridging effects of interatomic bonding, the κ of high-entropy tantalates is not necessarily lower than that of medium-entropy materials, suggesting that attempts to reduce κ 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-κ materials by regulating the inhomogeneity of interatomic bonding.