The vibrational properties of the type-I clathrates Rb8Sn44 square(2) and Cs8Sn44 square(2) (square 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. (C) 2008 Elsevier B.V. All rights reserved.