Résumé
We report a detailed study of specific heat, electrical resistivity, and thermal expansion in combination with inelastic neutron and inelastic x-ray scattering to investigate the origin of superconductivity in the two silicon clathrate superconductors Ba8Si46 and Ba24Si100. Both compounds have a similar structure based on encaged barium atoms in oversized silicon cages. However, the transition temperatures are rather different: 8 and 1.5 K, respectively. By extracting the superconducting properties, phonon density of states, electron-phonon coupling function, and phonon anharmonicity from these measurements, we discuss the important factors governing T-c and explain the difference between the two compounds.