  Atomic motion of guest atoms inside semiconducting clathrate cages is
considered as an important source for the glasslike thermal behavior.69Ga and
71Ga Nuclear Magnetic Resonance (NMR) 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. The 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 anharmonicity.
