Vibrational properties of tin clathrate materials

We have performed density functional calculations of the vibrational modes of three tin-based, cagelike clathrate materials: the pure type-I (Sn₄₆) and type-II (Sn₁₃₆) frameworks, and the donor/acceptor compensated compound Cs₈Ga₈Sn₃₈: We have also theoretically identified the infrared- and Raman-active modes in these materials and have computed their Raman spectra. In addition, we report measurements of the Raman spectrum of Cs₈Ga₈Sn₃₈ and compare the experimental spectrum with the theoretical one. By this means, we are able to unambiguously identify the low-frequency "rattling" vibrational modes in this material, which are due to the motion of the Cs atoms that are loosely bound in the Sn cages.