Guest host interaction and low energy host structure dynamics in tin clathrates

The two binary clathrates with vacancies (square) Rb8Sn44 square(2) and Cs8Sn44 square(2) have been examined using powder inelastic neutron scattering (INS). Rattling energies of Rb and Cs are found to be similar by both experiment and calculations, h omega(Cs) =h omega(Rb)vertical bar(Exp) = 0.98(1) and h omega(Cs) =h omega(Rb)vertical bar(Calc) = 1.0, despite the significant mass difference: m(Cs)/m(Rb) = 1.6, which shows that guest-host interaction is non-negligible for the studied system. For Rb8Sn44 square(2), a low energy phonon mode is observed at approximate to 3.5 meV, below the phonon mode which in the literature is attributed to the guest atom. The 3.5 meV mode is interpreted to have significant spectral weight of Sn host atoms based on temperature dependence and comparison with published theoretical phonon calculations. The record of low thermal conductivity of the tin clathrates can be attributed to the host structure dynamics rather than the guest atom rattling. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793081]