The dynamics of the guests in filled Germanium clathrates

In the filled gallium-germanium clathrates, R ₈Ga ₁₆Ge ₃₀, where R is Ba, Sr, or Eu, the guests are located in two large cages and are weakly bound to the crystalline clathrate framework. The caged guests exhibit a localized "rattling" vibrational mode that provides an efficient mechanism for reducing the thermal conductivity. Inelastic neutron scattering and nuclear inelastic scattering measurements have yielded the phonon density of states in R ₈Ga ₁₆Ge ₃₀', the line width of the localized vibrational modes is found to be an important parameter in determining the lattice thermal conductivity. Neutron diffraction studies on R ₈Ga ₁₆Ge ₃₀ have shown that the guests in the larger cage are located off-center, and it was proposed that their jumping about the four off-center locations is responsible for the observed glass-like thermal conductivity at temperatures below 10 K. The detection of such slow guest motion is challenging because the typical time and energy scales involved are ca. 4 ns and 1 μeV, respectively. We have studied the slow europium tunneling dynamics in Eu ₄Sr ₄Ga ₁₆Ge ₃₀ by both Mössbauer and microwave absorption spectroscopy.