Low-energy excitations of guest molecules in clathrates and the Boson peak

Abstract

The heat capacity, Cp, of seven ice clathrates, four β-quinol clathrates and one squarate clathrate containing rare gas atoms or molecules as guests in their cage-like structures at T<30 K has been analysed, and compared against that of empty β-quinol-clathrate, two channel-type thiourea clathrates containing polar molecules and four canonical glasses. Ethylene oxide–ice clathrate shows a peak in Cp/T3 at ca. 7.5 K, and argon–β-quinol clathrate at ca. 16 K. Other clathrates show an approach toward a Cp/T3 peak at 5<T<12 K. On the basis of their (i) Cp, (ii) Raman scattering, (iii) far infra-red absorption, and (iv) density of vibrational states data, it is shown that low-energy (2–8 meV) excitations are intrinsic to clathrates. Their features are remarkably similar to those of the Boson peak, which has been interpreted in terms of translational vibrations of 20–100 atoms group or of strongly cohesive nanometer size zones separated by softer zones in the heterogeneous glass structures. Since only translational (rattling) and/or rotational (torsional oscillations or librations) motions of atoms or molecules can occur in clathrate cages and channels, and nanometer size clusters cannot form, the similarity noted above indicates that the microstructural origin of the Boson peak in glasses may need reconsideration. Interaction between the guest and cage molecules explains satisfactorily neither the low-energy (2–8 meV) features of clathrates, nor its low-thermal conductivity relative to ice.