Low frequency vibrations in crystalline biphenyl: Model calculations and raman and neutron spectra

Vibrational frequencies of crystalline biphenyl have been calculated in the rigid phenyl approximation using the crystal structure at 110 K. The calculated results explain successfully the experimental infrared and Raman frequencies as well as the polarization data of the Raman bands at 80 K that have been observed in the present study for the first time. The frequency dispersion curves calculated in the b^* direction show that the coupling between the intramolecular CC torsion and the translational lattice vibrations changes drastically as the wave vector changes. The existence of a minimum frequency point located away from the Brillouin zone boundary indicates that the pulse transition near 40 K is a commensurate—incommensurate one in accordance with the results of neutron diffraction that have recently been reported. Incoherent inelastic neutron scattering spectra have also been investigated above and below the phase transition temperature.