Far-infrared vibrational properties of high-pressure high-temperatureC60polymers and theC60dimer

Abstract

We report high-resolution far-infrared transmission measurements of the $2+2$ cycloaddition ${\mathrm{C}}_{60}$ dimer and two-dimensional rhombohedral and one-dimensional orthorhombic high-pressure high-temperature ${\mathrm{C}}_{60}$ polymers. In the spectral region investigated $(20--650 {\mathrm{cm}}^{\ensuremath{-}1}),$ we see no low-energy interball modes, but symmetry breaking of the linked ${\mathrm{C}}_{60}$ balls is evident in the complex spectrum of intramolecular modes. Experimental features suggest large splittings or frequency shifts of some ${I}_{h}{\mathrm{C}}_{60}\ensuremath{-}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{d}$ 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 ${I}_{h}{\mathrm{C}}_{60}$ symmetries of observed modes. Our calculations show unprecedentedly large downshifts of ${T}_{1u}(2)\ensuremath{-}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{d}$ modes and extremely large splittings of other modes, both of which are consistent with the experimental spectra. For the rhombohedral and orthorhombic polymers, the ${T}_{1u}(2)\ensuremath{-}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{d}$ mode that is pola along the bonding direction is calculated to downshift below any ${T}_{1u}(1)\ensuremath{-}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{d}\mathrm{r}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d}$ modes. We also identify a previously unassigned feature near $610 {\mathrm{cm}}^{\ensuremath{-}1}$ in all three systems as a widely split or shifted mode derived from various silent ${I}_{h}{\mathrm{C}}_{60}$ vibrations, confirming a strong perturbation model for these linked fullerene structures.