Utilization and conversion of waste heat into useful electrical power is emerging as a promising source of energy. The efficiency of energy conversion, however, is low due to several factors with a thermal conductivity as one of the main factors. One of the strategies to enhance efficiency is by lowering thermal conductivity by introducing rattling atoms into materials with cage-like structures, such as skutterudites. The exact role played by these rattlers in the cage system is not clearly understood until now. In our work, we have synthesized skutterudite cage structures without and with single and double rattlers. To elucidate the role of these rattler atoms on thermal conductivity, we would like to use inelastic neutron scattering as this is ideally suited to probe the phonon density of states.