RAMAN-SCATTERING FROM VIBRATIONAL-MODES IN SI-46 CLATHRATES

Room-temperature Raman-scattering spectra are reported for the type-II superconductors MxBaySi46 (M = Na,K) that were recently shown to exhi bit T-c's similar to 3.5 K. These spectra an compared to those of Na8S i46 and K7Si46 clathrates that exhibit normal metallic behavior down t o 2 K. In the Si-46 system, fifteen of the eighteen Si related first-o rder Raman frequencies predicted by group theory have been detected, a nd the frequencies are found to be sensitive to the particular dopants . The Raman linewidths observed for the MxBaySi46 system are comparabl e to those observed for Na8Si46 and K7Si46. The data, taken collective ly, suggest that the line broadening in the metallic Si clathrates is due to important contributions from both the electron-phonon interacti on as well as from a random filling of the Si cages and charge-transfe r disorder.