Phonon density of states of silicon clathrates: Characteristic width narrowing effect with respect to the diamond phase

Abstract

The phonon density of states of different silicon clathrate phases is measured by inelastic neutron scattering. ${M}_{x}@\mathrm{S}\mathrm{i}\ensuremath{-}34$ and ${M}_{x}@\mathrm{S}\mathrm{i}\ensuremath{-}46$ 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.