The Dynamics of the Guests in Filled Germanium Clathrates
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The Dynamics of the Guests in Filled Germanium Clathrates R. P. Hermann,1,2 F. Grandjean,1 V. Keppens,2 W. Schweika,3 G. S. Nolas,4 D. G. Mandrus,5 B. C. Sales,5 H. M. Christen,5 P. Bonville,6 and Gary J. Long7 1 Department of Physics, B5, University of Liège, B-4000 Sart-Tilman, Belgium 2 Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200 3 Institut für Festkörperforschung, Forschungszentrum Jülich, Jülich, Germany 4 Department of Physics, University of South Florida, Tampa, FL 3362 5 Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 6 CEA-Saclay, Service de Physique de l'État Condensé, F-91191 Gif-sur-Yvette, France 7 Department of Chemistry, University of Missouri-Rolla, Rolla, MO 65409-0010 ABSTRACT In the filled gallium-germanium clathrates, R8Ga16Ge30, 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 R8Ga16Ge30; 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 R8Ga16Ge30 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 Eu4Sr4Ga16Ge30 by both Mössbauer and microwave absorption spectroscopy. INTRODUCTION The study of the dynamics of the caged guests in open structured compounds is a key to understanding the thermal conductivity of the phonon-glass electron-crystal thermoelectric materials that contain so-called “rattlers”.[1] The presence of the localized vibrational modes related to these “rattlers” has been extensively studied in the filled skutterudites.[2,3,4] Earlier studies have also demonstrated the presence of localized vibrational modes in hydroquinone and water clathrates.[5,6] The filled clathrates, R8Ga16Ge30, where R is Ba, Sr, or Eu, are another family of thermoelectric materials in which the dynamics of the guests trapped in the gallium and germanium clathrate cages is responsible for the low thermal conductivity.[7-10] The Ge and Ga framework atoms of the type-I clathrate, see Figure 1, are four-coordinate, as in the diamond structure, however, the insertion of the guest in the structure induces a deformation of the bonding angles.[11] The clathrate structures consist of stacks of different cages and the type-I clathrates studied herein are composed of smaller d
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