Ultralow lattice thermal conductivity of the fully filled skutteruditeYbFe4Sb12due to the flat avoided-crossing filler modes

We study the lattice thermal conductivity $(\ensuremath{\kappa})$ of fully filled skutterudites $\mathrm{YbFe}{}_{4}{\text{Sb}}_{12}$ and ${\text{BaFe}}_{4}{\text{Sb}}_{12}$ from first principles. The calculated $\ensuremath{\kappa}$ of ${\text{YbFe}}_{4}{\text{Sb}}_{12}$ is ten times lower than that calculated for $\mathrm{BaFe}{}_{4}{\text{Sb}}_{12}$, and much lower than any values ever reported for other skutterudites. The ultralow $\ensuremath{\kappa}$ of $\mathrm{YbFe}{}_{4}{\text{Sb}}_{12}$ is closely related to the flat Yb-dominated modes appearing in the frequency range from 5 to 7.2 rad/ps. The flat modes significantly increase the three-phonon scattering channels, which is reflected by the unique characteristics of weighted phase space and leads to much stronger anharmonic scattering of intermediate-frequency optical phonons. Although those flat modes are dominated by the Yb atoms, the filler-related anharmonic interaction does not play a role in the increased phonon scattering. This underlines the importance of the hybridization of the filler and the host matrix in those flat modes, which can be guaranteed by the avoided crossing with acoustic phonons. The depressed phonon spectrum mechanism, common to other skutterudites, also plays a role in the ultralow $\ensuremath{\kappa}$. Our work presents a reduction mechanism of $\ensuremath{\kappa}$ by the filler in cage-like structures such as skutterudites and clathrates, and demonstrates the nonuniqueness of the reduction mechanism in these materials.