The intermetallic clathrates first discovered in 1965  attracted attention of chemists and physicists due to the fascinating structural features, especially the formation of large cavities within the three-dimensional framework . These cavities may be also un-occupied (empty clathrates ). The coexistence of the different bond kinds (inhomogeneity of the bonding) is one of the reasons for reduced thermal conductivity and opens also the possibility to tune the charge carrier concentration, which makes these materials interesting for thermoelectric applications . The suitable combination of the electronic and phononic transport in clathrates for thermoelectric application was recognized and proven quite fast [5,6]. One of the challenges on the way to an application is the understanding of the low thermal conductivity of this family of materials. One possible mechanism is associated with the vibrations (‘rattling’) of the filler atoms within the cage-like crystal structure (e.g. [7,8]). Recently, the phonon filtering mechanism was proven by the inelastic neutron scattering experiments [9,10].
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