Raman scattering from vibrational modes in Si46 clathrates

Room-temperature Raman-scattering spectra are reported for the type-II superconductors MxBaySi46 (M = Na,K) that were recently shown to exhibit T-c's similar to 3.5 K. These spectra an compared to those of Na8Si46 and K7Si46 clathrates that exhibit normal metallic behavior down to 2 K. In the Si-46 system, fifteen of the eighteen Si related first-order Raman frequencies predicted by group theory have been detected, and the frequencies are found to be sensitive to the particular dopants. The Raman linewidths observed for the MxBaySi46 system are comparable to those observed for Na8Si46 and K7Si46. The data, taken collectively, suggest that the line broadening in the metallic Si clathrates is due to important contributions from both the electron-phonon interaction as well as from a random filling of the Si cages and charge-transfer disorder.