Far-infrared vibrational properties of high-pressure high-temperatureC60polymers and theC60

Abstract

We report high-resolution far-infrared transmission measurements of the 2ϩ2 cycloaddition C 60 dimer and two-dimensional rhombohedral and one-dimensional orthorhombic high-pressure high-temperature C 60 polymers. In the spectral region investigated (20-650 cm Ϫ1 ), we see no low-energy interball modes, but symmetry breaking of the linked C 60 balls is evident in the complex spectrum of intramolecular modes. Experimental features suggest large splittings or frequency shifts of some I h C 60 -derived modes that are activated by symmetry reduction, implying that the balls are strongly distorted in these structures. We have calculated the vibrations of all three systems by first-principles quantum molecular dynamics and use them to assign the predominant I h C 60 symmetries of observed modes. Our calculations show unprecedentedly large downshifts of T 1u ͑2͒-derived modes and extremely large splittings of other modes, both of which are consistent with the experimental spectra. For the rhombohedral and orthorhombic polymers, the T 1u ͑2͒-derived mode that is polarized along the bonding direction is calculated to downshift below any T 1u ͑1͒-derived modes. We also identify a previously unassigned feature near 610 cm Ϫ1 in all three systems as a widely split or shifted mode derived from various silent I h C 60 vibrations, confirming a strong perturbation model for these linked fullerene structures.

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

“…Similar closely spaced lines were also observed in the Raman 36 and IR 45 spectra of a high-pressure high-temperature tetragonal C 60 polymer, where intense modes are also visible at frequencies corresponding to the 38 and 48 meV modes. Densely spaced modes were also observed in the 50-100 meV energy range in the orthorhombic phase of pressure-polymerized C 60 (both in Raman 46 and IR 47 ) and in IR studies on orthorhombic RbC 60 and CsC 60 . 48,49 The point symmetry in these phases is D 2h , which contains more symmetry elements than the C 2h group of Li 4 C 60 .…”

Far-infrared vibrational properties of tetragonalC60polymer

“…18. The method was applied successfully to fullerene molecules, 19 polymerized fullerenes, 16,18,20,21 and carbon solids. 22 Our calculated vibrational frequencies are known to be reliable, especially in our frequency region of interest; 200-600 cm Ϫ1 ͑Ref.…”

“…On the other hand, as discussed in Refs. 16,18, and 24, our first-principles IR strengths are not fully reliable for all modes, and as a consequence we have not used them here.…”

“…The T polymer is an intermediate case, the ball being distorted by four 2ϩ2 connections. In our earlier IR study 16 of O and R polymers, we found that modes of primarily T 1u (2) parentage, polarized in the stretched directions, are strongly downshifted to a frequency below that of any modes with substantial T 1u (1) character. The fact that this large downshift ͑more than 50 cm Ϫ1 ) occurs even for the O polymer, which is the least distorted of these HTHP polymers, is already an indication that the polymer-ball distortions are too large to permit the use of a weak perturbation model for the assignment of vibrations.…”

“…17 QMD is based on the local density and pseudopotential approximations; details of the method were described briefly in Ref. 16, and extensively in Ref. 18.…”