Phonon dispersion of the external and low-frequency internal vibrations in monoclinic tetracyanoethylene at 5K

Abstract

Phonon dispersion relations have been determined using neutron coherent inelastic scattering in the monoclinic phase of tetracyanoethylene supercooled to 5K, for 24 low-frequency branches comprising 12 external and 12 internal vibrations along the symmetry directions a*, b* and c*. Theoretical predictions have been made using an extension of the external mode formalism in which only the seven low-frequency internal vibrations (one just out of experimental range) of the free molecule are considered along with the external vibrations. The various experimental results from neutron, Raman and infrared spectroscopy agree satisfactorily with the theoretical predictions. Appreciable dispersion and Davydov splitting are found for the 'internal' phonons and the eigenvectors in general contain both internal and external character. Model improvements are thought to be possible only by the introduction of Coulomb interactions between these highly polarisable molecules. The dispersion curves show no evidence of phonon instability which may be related to the solid state phase transition.