Highly efficient thermal insulation in crystalline weberites RE3NbO7 (RE=La, Nd, Sm, Eu, Gd) with glass-like thermal conductivity

Thermal energy has been considered to propagate by wave-like phonons with well-defined wave vectors in crystalline materials. In this work, we demonstrate a completely different thermal conduction scenario in crystalline rare earth niobates, RE3NbO7 (RE=La, Nd, Sm, Eu, Gd), having ultralow thermal conductivities of 1.0-1.5 Wm-1K-1. Especially, the rare earth niobates exhibit glass-like temperature dependence of thermal conductivity from room temperature to 1273 K, indicating significant suppression of both phonon and radiation heat transfer. According to Cahill model based on amorphous theory, minimum thermal conductivity of rare earth niobate was calculated. It is shown that the glass-like thermal conductivity result from a wide range of amorphous solids due to phonon scattering by multiple lattice imperfections. Furthermore, phonon mean free path of RE3NbO7 is comparable to the atomic spacing of crystal indicating that energy carried by phonons can only transmission between a few atomic spacings leading to the ultralow thermal conductivity. The mechanical properties of rare earth niobates are also discussed.