  Engineering of thermoelectric materials requires an understanding of thermal
conduction by lattice and electronic degrees of freedom. Filled skutterudites
denote a large family of materials suitable for thermoelectric applications
where reduced lattice thermal conduction attributed to localized low-frequency
vibrations (rattling) of filler cations inside large cages of the structure. In
this work, a multi-wavelength method of exploiting X-ray dynamical diffraction
in single crystals of CeFe$_4$P$_{12}$ is presented and applied to resolve the
atomic amplitudes of vibrations. The results suggest that the vibrational
dynamics of the whole filler-cage system is the actual active mechanism behind
the optimization of thermoelectric properties.
