Phonon Dynamics of Type-I Clathrate Sr8Ga16Ge30 Studied by Inelastic Neutron Scattering

The phonon dynamics of type-I clathrates Sr8Ga16Ge30 have been studied at room temperature by inelastic neutron scattering. Optical phonons associated with large vibrations of Sr atoms in large tetrakaidecahedral cages, (Ga,Ge)(24), are observed at a relatively low energy of E similar to 4 meV. Analysis based on a Born-von Karman force model shows that the longitudinal force constants between the Sr atoms and the oversized cages has a small value of about 0.008 mdyn/angstrom. The results indicate that the Sr atoms are very loosely bound to the surrounding oversized cages.