The phonon density of states of different silicon clathrate phases is measured by inelastic neutron scattering. $M_x@\mathrm{S}\mathrm{i}-34$ and $M_x@\mathrm{S}\mathrm{i}-46$ clathrates with different alkaline $(M=\mathrm{N}\mathrm{a},\mathrm{}\mathrm{K})$ and composition x are investigated. A width-narrowing effect is observed on the phonon spectra with respect to the diamond silicon structure. The effect of the alkali atom-silicon atom coupling is seen demonstrating the strong interaction between the alkali- atom and the host silicon cage. Such specific properties of clathrate lattices are interpreted in term of local icosahedral symmetry.

• Received 21 October 1998

DOI:https://doi.org/10.1103/PhysRevB.59.10099