Atomic motion of guest atoms inside semiconducting clathrate cages is considered
an important source for glasslike thermal behavior. 69Ga and 71Ga nuclear
magnetic resonance studies on type I Ba8Ga16Sn30 show a clear low-temperature
relaxation peak attributed to the influence of Ba rattling dynamics on the
framework-atom resonance, with a quadrupolar relaxation mechanism as the
leading contribution. Data are analyzed using a two-phonon Raman process,
according to a recent theory involving localized anharmonic oscillators.
Excellent agreement is obtained using this model, with the parameters
corresponding to a uniform array of localized oscillators with very large
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