Far-infrared vibrational properties of tetragonalC60polymer

Abstract

We report high-resolution far-infrared transmittance measurements and quantum-molecular-dynamics calculations of the two-dimensional tetragonal ͑T͒ high-temperature/high-pressure C 60 polymer, as a complement to our previous work on the C 60 dimer, and the one-dimensional orthorhombic ͑O͒ and two-dimensional rhombohedral (R) C 60 polymers ͓V. C. Long et al., Phys. Rev. B 61, 13 191 ͑2000͔͒. The spectral features are assigned as intramolecular modes according to our quantum-molecular-dynamics calculations. In addition, we determine the I h C 60 parent symmetry of each polymer vibrational mode by expanding the calculated polymer eigenvectors in terms of our calculated eigenvectors for I h C 60 . We find that many of the T-polymer vibrational modes are derived from more than one I h C 60 parent symmetry, confirming that a weak perturbation model is inadequate for these covalently bonded C 60 balls. In particular, strongly infrared-active T-polymer modes with frequencies of 606 and 610 cm Ϫ1 are found to be derived from a linear combination of three or more I h C 60 parent modes. As in the O and R polymers, modes of the T polymer with substantial T 1u (2) character, which are polarized in the stretched directions, are found to have large downshifts. Finally, in our comparison of theory with experiment, we find indications that the in-plane lattice of the T polymer may not actually be square.

Low Band Gap and Ionic Bonding with Charge Transfer Threshold in the Polymeric Lithium Fulleride Li4C60

“…37 Several polymer modes of tetragonal C 60 are indeed found to derive from linear combinations of two, three, or more parent icosahedral modes. 45 The closely related system Na 4 C 60 (C i symmetry with space group I2/m) 10 has a lower point symmetry with respect to Li 4 C 60 , while maintaining the inversion symmetry. Also, in Na 4 C 60 all vibrational modes are nondegenerate.…”

Far-infrared vibrational properties of linearC60polymers: A comparison between neutral and charged materials

“…Three other modes, at 523, 543, and 613 cm Ϫ1 , have considerable mixing of T 1u (1), T 1u (2), and H u (2) I h C 60 parent symmetries. We note that, in our previous work, even greater collective character was found for modes of the twodimensional T and R polymers; 22 the degree of mode mixing seems to correlate with on-ball distortion. Finally, the feature at 520 cm Ϫ1 is assigned to a T 1u (2)-derived mode which is polarized in the stretched direction.…”

“…1 of our earlier work on the T polymer. 22 The most likely ''underpolymerized'' defects are ''chain-end'' defects, i.e., the C 60 balls which occur at the end of an oligomer. 40 We have simulated two structures which contain these types of C 60 balls, the dimer of Fig.…”

“…This finding is ubiquitous among the HTHP polymers. 21,22 The unassigned features in the HTHP O-polymer spectrum are small compared with the typical size of the assigned features and are assumed to be due to remaining ''overpolymerized'' and ''underpolymerized'' defects in our O-polymer sample. Several of these features are likely to be due to chain-end defects ͑for example, the weak feature at 564 cm Ϫ1 ), but any detailed assignments at this time would be highly speculative.…”

“…18,19 The reduced symmetry of the C 60 balls in the polymerized materials results in a much larger number of infrared-and Raman-active modes. 9,20 In our previous work, 21,22 we assigned the majority of far-infrared spectral features of the neutral two-dimensional polymers with the support of quantum molecular dynamics ͑QMD͒ calculations. For the O-polymer material, however, our mode assignments were not reliable enough to permit a detailed comparison with those of the analogous charged polymer, mainly because of the multiphase nature of the sample, which resulted in a large number of unassigned features and ambiguous mode assignments.…”