A brief introduction to the thermoelectric effects, the studied materials, and the experimental methods is given in the ﬁrst chapter. The introduction is not intended to be exhaustive, but only to summarize important basic information for the reader. The introduction is followed by four chapters dedicated to detailed experimental studies of the lattice dynamics in selected thermoelectric Zintl phases.
First, the lattice dynamics in the unﬁlled and ﬁlled skutterudites FeSb3 , CoSb3 , and YbFe4Sb12 were studied by nuclear inelastic scattering, inelastic neutron scattering, and several macroscopic methods. These studies reveal that FeSb3 exhibits softer Sb bonds than CoSb3 , that the density of phonon states is shifted towards lower energies and the velocity of sound is lower in FeSb3 as compared to CoSb3 . It appears thus that the soft [Fe4Sb12]
framework dynamics might play an important role in the thermoelectric properties of ﬁlled skutterudites. The observed anomalous temperature dependence of the elastic constants
and the rearrangement of the spectral weight of the Yb phonon states in YbFe4 Sb12 can be explained by a change of the Yb valence state with the temperature.
Second, the lattice dynamics in the Sr8 Ga16 Ge30 clathrate was investigated by inelastic neutron scattering measurements on a single crystal. We found that several mechanisms contribute to the low thermal conductivity in this system and that the reduction of the heat capacity contribution to the thermal conductivity plays a signiﬁcant role to the low thermal conductivity, besides the reduction in the phonon lifetime and the phonon group velocity that is related to the guest atom.
Third, the lattice dynamics of the Zintl phase Yb14 MnSb11 was studied by inelastic neutron scattering and nuclear inelastic scattering measurements. All phonon modes of these
complex crystal systems are in a narrow energy range below ∼25 meV and the Debye temperature, the velocity of sound and the mean force constants are small compared to those of other thermoelectric materials such as Zn4Sb3. By comparing the lattice dynamics in Yb14MnSb11 and Zn4Sb3 different mechanisms which lead to the low thermal conductivity in Zintl phases have been identiﬁed. Between 300 and 1200 K no softening of the velocity of sound in Yb14MnSb11 was observed by temperature dependent inelastic neutron scattering measurements, which is in line with its large melting temperature.