Lattice dynamics and thermal conductivity of skutteruditesCoSb3andIrSb3from first principles: WhyIrSb3is a better thermal conductor thanCoSb3

Abstract

Materials with heavier atomic masses usually possess lower lattice thermal conductivity ($\ensuremath{\kappa}$). The reported $\ensuremath{\kappa}$ of IrSb3 skutterudite is about 35% higher than that of CoSb3, despite Ir being much heavier than Co. We study the lattice dynamics and $\ensuremath{\kappa}$ of $\mathrm{CoSb}{}_{3}$ and $\mathrm{IrSb}{}_{3}$ from first principles. We unveil the physical reasons for the difference in $\ensuremath{\kappa}$ by comparing all the influential factors: phonon velocities, anharmonicity characterized by the third-order interatomic force constants, the weighted phase space $W$, and the atomic mass. We find the increased mass from Co to Ir is ultimately the dominant factor resulting in the increase of $\ensuremath{\kappa}$ in $\mathrm{IrSb}{}_{3}$, and the other factors tend to reduce $\ensuremath{\kappa}$. Larger mass leads to smaller thermal displacements causing weaker anharmonic scattering. Our work provides deeper insight to understand the correlation of $\ensuremath{\kappa}$ of systems sharing the same crystal structure. We also find that the decreases in acoustic phonon frequencies and Debye temperature in $\mathrm{IrSb}{}_{3}$ are almost entirely due to the mass increase from Co to Ir.