The crystal structure of two new cubic phases in the silicon-sodium system have been solved from their x-ray diffraction patterns. Both structures are of the clathrate type found for gas hydrates, consisting of tetrahedral networks which are combinations of pentagonal dodecahedra with 14-face polyhedra in one case and with 16-face polyhedra in the other case. There is strict correspondence between the silicon positions and the oxygen positions of the hydrate structures. For one compound, Na(8)Si(46), the centers of all polyhedra are occupied by sodium atoms. For the other compound, there occurs only partial occupancy of the polyhedral cages.

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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

Several doped type-I silicon clathrates (Na 8 @Si 46 , K 8 @Si 46 , Ba 8 @Si 46 and I 8 @Si 46 ) have been investigated both experimentally by inelastic neutron scattering and x-ray-absorption spectroscopy and theoretically by ab initio calculations. We find that Ba atoms have a stronger coupling with the host lattice than Na or K ones. We show that in the superconductor Ba 8 @Si 46 the coupling is effectively done by host acoustic phonons exciting guest acoustic modes. These features suggest that the host-guest vibrational coupling is a crucial parameter to explain the appearance of superconductivity in addition to the density of states near the Fermi level.