T well described by a Lorentzian lineshape for both polarizations HH(||, ||) and HV(||,1) [11.3,114], metal M concentration x for three dominant modes in MXC60 (M = K, Rb, Cs) is presented. The data in Fig. 11.24 come from several publications of the Kentucky, Vienna, and AT&T research groups [11.19,112,113]. Normal mode displacements for two of the three mode eigenvectors are also shown in the figure (see also Figs. 11.2 and 11.3).
The softening of the 1469 cm-1 tangential Ag(2) mode by alkali metal doping is often used as a convenient method to characterize the stoichiom-etry x of K^C^ samples. This mode in MXC60 is downshifted by ~6 cirr'/M atom, and since all end-point M6C60 compounds (M = K, Rb, Cs) exhibit approximately the same value for this mode frequency (1432 cm-1), it is reasonable to expect that a downshift of ~6 cm_1/M atom for the Ag(2) mode frequency is approximately applicable also for both K and Rb. It is found that the frequency downshifts of the Ag(2) mode for x = 1, 3, 4, and 6 follow a linear dependence on x quite well [11.118]. The radial Ag(l) mode, on the other hand, stiffens slightly with increasing x due to competing effects, associated with a mode softening arising from the charge transfer effect (similar to the situation for the tangential modes) and a larger mode stiffening effect due to electrostatic interactions between the charged C60 molecule and the surrounding charged alkali metal atoms, as their atomic separations change during a normal mode displacement [11.117].
Mode broadening occurs with increasing x, and this broadening is most pronounced near x ~ 3 in the metallic phases of M3C60. This line broadening has in part been attributed to enhanced electron-phonon coupling, and
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