Inelastic neutron scattering measurements on Ba8Ga16Ge30 and Sr8Ga16Ge30 indicate the presence of soft local modes with Einstein oscillator energies of 4.9±0.1 and 4.4±0.1meV, modes which are associated with the lowest “rattling” mode of the Ba(B) and Sr(B) guests, respectively. Nuclear inelastic scattering measurements on Eu8Ga16Ge30 yield Einstein oscillator energies of between 3.0±0.3 and 7.5±1.2meV for the different modes of the Eu guests. Further, the resonant character of the measurements on Eu8Ga16Ge30 reveals, without question, that neither of the Eu guests exhibit any vibrational modes above 9meV.
R. P. Hermann et al., "Neutron and Nuclear Inelastic Scattering Study of the Einstein Oscillators in Ba-, Sr-, and Eu-filled Germanium Clathrates," Physical review B: Condensed matter and materials physics, vol. 72, no. 17, American Physical Society (APS), Nov 2005.
The definitive version is available at https://doi.org/10.1103/PhysRevB.72.174301
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