Rattling guest atoms in Si, Ge, and Sn‐based type‐II clathrate materials

Abstract

We have studied the vibrational properties of some of the compounds based on the Type II silicon, germanium, and tin-based clathrate materials using LDA electronic structure methods. The lattices in these framework materials have open cages which can contain weakly bound guest impurities, and these guests produce local ("rattling") vibrational modes. Such modes may scatter the extended, heat carrying acoustic vibrational modes of the framework, potentially reducing the thermal conductivity. We present results for the vibrational spectra of the Type II clathrate compounds Na 16 Cs 8 Si 136 , Na 16 Cs 8 Ge 136 , and Cs 24 Sn 136 . We discuss trends in the vibrational spectra of the framework host material and of the localized, rattling frequencies of the guest modes as the host changes from Si to Ge to Sn.

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

“…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 phase fractions of (clathrate-I) Na 8 Si 46 and (diamond structure) α-Si impurity phases commonly observed [10] in Na x Si 136 specimens were minimized in the specimens reported on here (approx. 2 wt% Na 8 Si 46 and 1 wt% α-Si for the Na 3 Si 136 specimen, and approx.…”

“…The description of Na@Si 28 at the 8b site implies relatively high symmetry, such that the broadening of the mode associated with this guest can be interpreted as an indication of a shorter mean lifetime of these phonons. Although to our knowledge no theoretical reports of phonon spectra and/or lattice dynamics exist yet for Na x Si 136 , our conclusion appears to be supported by the single rattler frequency found from DFT calculations for other alkali guests in the pseudospherical Si 28 cage [8]. Shorter mean lifetimes may be indicative of an enhanced collision rate with acoustic phonons, and low lattice thermal conductivity can be expected in such cases.…”

“…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).…”

“…scattering [7], and density functional theory [8] investigations have indicated rattling behavior in the stoichiometric (i.e. full guest occupation) A 8 Na 16 E 136 clathrates (A = Cs, Rb; E = Si, Ge).…”

Synthesis, Crystal Structure, and Transport Properties of Na22Si136

“…Large-amplitude, localized vibrations of these guests in the relatively oversized framework cages can potentially result in strong scattering of the heat-carrying phonons. [3][4][5][6][7][17][18][19] This aspect, coupled with the favorable electrical properties that semiconducting type II clathrates are expected to possess, forms the impetus for investigation of these materials for potential thermoelectric applications.…”