Info

sured, and some typical results are shown in Table 11.9. The high-frequency modes appear to harden with increasing pressure, in contrast to the low-frequency modes, some of which harden while others soften, showing less of a clear trend. Above 22 GPa, the characteristic Raman spectrum for Qq is no longer observed. On the basis of the vibrational spectra, an irreversible transition to an amorphous carbon phase occurs above this pressure [11.149]. The Gruneisen parameters (which describe how the frequency of a given Raman mode scales with the volume) are very small. These Gruneisen parameters are compatible with there being very little change in intramolecular bond length for a change in intermolecular spacing of crystalline C60

Similar to the spectral characteristics of the polymerized films (see §11.8), the spectral features for the high pressure-high temperature phases for C60 (see §7.3) also show more lines in the infrared and Raman spectra, as compared with crystalline C60 (see Figs. 11.37 and 11.38). Both pressure-induced forms of CM, namely fcc(pC60) and rh(p C60) (see §7.3), show structure in the infrared spectra in the 700-800 cm-1 range (see Fig. 11.37), similar to the behavior of polymerized films (see Fig. 11.33). The downshift of the Flu(A) mode from 1428 cm-1 in conventional fee C60 to 1422 cm-1 in fcc(pC60), to 1383 cm-1 for rh(pC60), the line broadening, and the appearance of multiple spectral lines in the infrared spectra all suggest molecular distortions, changes in bonding, and changes in charge distribution in the pressure-induced phases of C60 relative to conventional crystalline CM [11.160]. The Raman spectra for the pressure-induced phases (see Fig. 11.38) [taken at a low power level (2 W/cm2) to avoid polymerization effects] likewise show broadened features, rich spectra containing many spectral features, and a downshift of the Ag(2) mode from 1468 cm-1 in fee C60, to 1457 cm-1 in fcc(pC60), and to 1447 cm"1 in rh(pC60). These features in the Raman spectra further corroborate the indication that the C60

Was this article helpful?

0 0

Post a comment