Anharmonicity in partially filled skutterudites YbxCo4Sb12

We report an experimental investigation of Yb filler ion dynamics in partially filled skutterudite samples YbxCo4Sb12 (x=0.26 and x=0.4). From extended x-ray absorption fine structure (EXAFS) measurements at the Yb L2-edge, the mean-square displacements of the Yb fillers were extracted over a wide temperature range from 10 to 500 K. We found that the low-temperature dynamics of Yb fillers are described well using a harmonic oscillator approximation with Einstein temperatures of 70±5 K for both samples. This finding is consistent with the observed low-energy phonon mode at 5.3(5) meV in the inelastic x-ray scattering (IXS) data. However, the temperature dependence deviates from the harmonic approximation at high temperatures due to significant cubic anharmonicity, which was revealed in the EXAFS cumulant analysis. Our study also demonstrates that complementary information on filler dynamics in skutterudites can be acquired using EXAFS and IXS.