Mechanism of amorphous-like thermal conductivityin binary oxide Yb₃TaO₇

The materials with low thermal conductivity (k) are both fundamentally interesting and technologicallyimportant in applications relevant to thermal energy conversion and thermal management, such asthermoelectric conversion devices, thermal barrier coatings, and thermal storage. Therefore, understanding thephysical mechanisms of glass-like heat conduction in crystalline materials is essential for the development anddesign of low-k materials. In this work, the microscopic phonon mechanism of glass-like low k in binary simplecrystal Yb3TaO7 with fluorite structure is investigated by using the equilibrium molecular dynamics, phononspectral energy density, and lattice dynamics. Meanwhile, the weberite-structured Yb3TaO7 is also mentionedfor comparison. The calculated k indicates that fluorite Yb3TaO7 has a glass-like low k while weberite Yb3TaO7has a crystal k. Such a low k in fluorite Yb3TaO7 is mainly due to the large difference in interatomic forcebetween O-Yb and O-Ta. This different atomic bonding can significantly soften the phonon mode and thus limitphonon transport. To further describe the microscopic phonon thermal conduction, the single-channel modelbased on the phonon gas model is first used to calculate the total k. However, the single-channel modelsignificantly underestimates the k, suggesting the presence of non-normal phonons in Yb3TaO7. Based on this,vibrational mode decomposition is conducted throughout the entire phonon spectrum of fluorite- and weberite-type Yb3TaO7. It is found that most modes in fluorite Yb3TaO7 fall in the Ioffe-Regel regime and exhibit astrongly diffusive nature. Such diffusive modes cannot be described by the phonon gas model. Based on thedecomposed phonon modes, the dual-channel model involving diffusive mode and propagating mode is used todescribe the phonon thermal conduction, by which the obtained results accord well with the experimentalvalues. The vast majority (> 90%) of heat in fluorite Yb3TaO7 is found to be transported by diffusive modesrather than propagating modes. Consequently, the k of fluorite Yb3TaO7 increases with temperature rising,exhibiting a unique glass-like nature. In particular, contrary to conventional wisdom, the optical phonon modein fluorite Yb3TaO7 plays a significant or even decisive role in thermal conduction, which could serve as a newphysical factor to adjust k in solid materials. Overall, the new understanding of the link between chemicalbonding and glass-like k can contribute to the development and design of low-k materials