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 @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

A study of low-energy guest phonon modes in clathrate-II NaxSi136(x= 3, 23, and 24)

“…The composition of both specimens was further analyzed using neutron prompt gamma activation at the PGAA spectrometer [11] (FRM II, Munich). The normalized Na content from this technique was estimated to be 4.9(5) and 22.9(5) for the Na 3 Si 136 and Na 23 Si 136 samples, respectively, in qualitative agreement with the Rietveld analysis.…”

“…2 wt% Na 8 Si 46 and 1 wt% α-Si for the Na 3 Si 136 specimen, and approx. 3 wt% Na 8 Si 46 for Na 23 Si 136 specimen as determined by Rietveld refinement), so that any scattering contribution from these phases to the INS spectra would be minimized. The composition of both specimens was further analyzed using neutron prompt gamma activation at the PGAA spectrometer [11] (FRM II, Munich).…”

“…Specimen composition was determined by Rietveld analysis, which established that in Na 3 Si 136 only the larger Si 28 cages are (partially) occupied and the Si 20 cages remain empty, while for Na 23 Si 136 the Si 28 cages are fully occupied and the Si 20 cages are almost fully occupied. This is in agreement with the observation that for x < 8, Na almost exclusively occupies the Si 28 cages; the Si 20 cages show significant occupation only after all Si 28 cages are completely filled [10].…”

“…The inelastic scattering from Na 3 Si 136 and Na 23 Si 136 powder at 298 K was measured on the cold neutron timeof-flight instrument FOCUS (SINQ, Villigen). The neutron wavelength was 4Å and the phonon density of states (PDOS) was calculated in the incoherent scattering approximation from the scattering integrated over all detectors after subtraction of the empty container contribution.…”

“…In either case, at elevated temperatures thermal excitation will cause Na to dynamically explore the large volume of the Si 28 cage, in agreement with the strong temperature dependence of U eq for the guest atom. Inelastic neutron scattering (INS) experiments on Na 3 Si 136 and Na 23 Si 136 , which we now discuss, offer further insight into the physical nature of the Na disorder for Na@Si 28 .…”