"Identified as a belt mode.

"Identified as a belt mode.

modes observed for the M6C70 compounds. Furthermore, the Raman spectra for K^Qq, Rb6C70, and Cs6C70 show many common features [11.154], as do the corresponding M6C60 Raman spectra [11.3,15,19], as previously noted. To aid with the mode identification, the calculated mode frequencies for M6C70 (see Table 11.8) could be used [11.147], but the agreement is only fair between the observed and predicted mode frequencies. The observed Raman mode frequency shifts for the M6C70 films relative to C70 show a strong correlation with the corresponding behavior of the M6C60 films relative to C60 [11.154],

In addition, the Raman spectra of the maximum-conductivity K4C70 phase were investigated to provide further evidence for enhanced electron-phonon coupling for a half-filled band (see §12.7.5). The experimental results for K4C70 (Fig. 11.32) show only a small amount of line broadening [11.148] relative to the behavior in the half-filled tlu band compound K3C60. In addition, a comparison between the Raman spectra of K^C^ and C70 (see Fig. 11.32) allows identification of the experimental frequency shifts of the various modes upon alkali metal doping. Assuming that the electron-phonon coupling strength is comparable to the Coulomb repulsion, the i i i i-1 i r

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