  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 K 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 Tc and explain the
difference between the two compounds.
