Phonons and electron-phonon interaction in halogen-fullerene compounds

Abstract

We have investigated the optical spectra of different halogen-fullerene compounds: ${\mathrm{C}}_{60}{\mathrm{I}}_{4\ensuremath{-}x},$ ${\mathrm{C}}_{70}{\mathrm{I}}_{2},$ ${\mathrm{C}}_{60}{\mathrm{Br}}_{24},$ ${\mathrm{C}}_{60}{\mathrm{Cl}}_{24},$ and ${\mathrm{C}}_{70}{\mathrm{Cl}}_{17}.$ Two types of carbon-halogen bonding have been established: (a) ${\mathrm{C}}_{60}{\mathrm{I}}_{4\ensuremath{-}x}$ and ${\mathrm{C}}_{70}{\mathrm{I}}_{2}$ compounds are formed by a ${\mathrm{C}}_{60}$ or ${\mathrm{C}}_{70}$ molecule sublattice and an ${\mathrm{I}}_{2}$ molecule sublattice that weakly interact via van der Waals forces; (b) ${\mathrm{C}}_{60}{\mathrm{Br}}_{24},$ ${\mathrm{C}}_{60}{\mathrm{Cl}}_{24},$ and ${\mathrm{C}}_{70}{\mathrm{Cl}}_{17}$ compounds are characterized by covalent bonds between C and Br/Cl atoms. We have studied in detail the resonance effects in ${\mathrm{C}}_{60}{\mathrm{Cl}}_{24}$ using the methods of Raman scattering, infrared absorption, and absorption in the visible region. The effect originates from the interactions between the phonon subsystem and the electron band at 2.33 eV and manifests itself in a resonant enhancement of the Raman line intensities and in the repetition of the phonon and the luminescence spectra shifted by the frequency of Raman-active phonon at $1508{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}.$ The group-theory analysis of phonon symmetries in rigid and nonrigid ${\mathrm{C}}_{60}{\mathrm{Br}}_{24}$ and ${\mathrm{C}}_{60}{\mathrm{Cl}}_{24}$ crystals has been performed.