Resonant Oscillations in Multiple-Filled Skutterudites

Partially filled skutterudites were investigated by ultrafast spectroscopy using a femtosecond laser system, which excites resonant oscillations due to the host–guest interactions. To investigate the effect of individual guest elements on phonon-mediated thermal transport, four skutterudite samples were studied: three samples partially filled with Ba, Yb, and La, respectively, and a fourth with a combination of these three elements. The spectrum of the oscillations in the transient thermoreflectance signal was analyzed by Fourier transformation. Comparison with the Raman spectra shows that different guest elements cause resonant oscillations with different frequencies, which can scatter phonons in different spectrum spans. This further demonstrates that multiple guest elements can scatter a wider spectrum of phonons than a single guest element at similar filling fractions, which results in lower lattice thermal conductivity (jL) in skutterudites. These findings are consistent with thermal conductivity measurements reported previously.