Raman spectroscopy study of type-I clathrates A8Sn44□2 (A = Rb, Cs) and Rb8Hg4Sn42

The vibrational properties of the type-I clathrates Rb8Sn44□2 and Cs8Sn44□2 (□ denotes framework vacancy) were investigated by Raman spectroscopy. The rattling vibrations of the guest Rb and Cs atoms occur in a low-wavenumber region 25–40 cm−1, whereas the vibrations of the Sn framework occur at wavenumbers 44–170 cm−1. Vacancy-induced Raman bands around 70 cm−1 have been observed for the first time and identified by comparing with the experimental Raman spectrum of vacancy-free Rb8Hg4Sn42 and the phonon density of states (PDOS) for hypothetical Cs8Sn46. The influence of the polyhedra symmetry on the vibrational modes is discussed.