Raman Spectra of ¹³C-Substituted and Li-Intercalated Fullerenes

Abstract

Raman spectra of both C₆₀ and C₇₀ fullerenes are at the present time well known and understood on both experimental and theoretical points of view. We have extended these studies to ¹³C substituted samples (82% substitution). As expected, an overall downshift of the Raman frequencies is observed, in good agreement with the values predicted from the total mass change of the molecules. In parallel studies carried out on the same samples intercalated with K at the superconducting level (K₃C₆₀), a significant shift in the T_c transition temperature has been measured. The isotope effect is therefore discussed in the framework of the BCS theory of superconductivity and compared to other published results.

We have also investigated the Raman spectra of C₆₀ doped with Li by a chemical method, according to a procedure which involves the use of organo-alkali compounds. Results are consistent with a charge transfer from Li atoms to fullerene molecules. Upon annealing, up to 250°C, these compounds present a desorption or a diffusion of Li atoms, a part of the system going back to neutral crystalline C₆₀, as evidenced by Raman, X-rays and NMR experiments.