The phonon density of states of different silicon clathrate phases is measured by inelastic neutron scattering. M x @Si-34 and M x @Si-46 clathrates with different alkaline (M ϭNa, 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 alkaliatom and the host silicon cage. Such specific properties of clathrate lattices are interpreted in term of local icosahedral symmetry. ͓S0163-1829͑99͒02415-7͔ PHYSICAL REVIEW B 15 APRIL 1999-I VOLUME 59, NUMBER 15 PRB 59
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