Lattice Dynamics, Raman Spectra and Soft Mode in 4-4'-Dichlorodiphenylsulfone

Abstract

An intermolecular semiempirical 6-exp potential model has been used to calculate the lattice dynamics of 4-4'-dichlorodiphenylsulphone allowing for molecular flexibility with three internal degrees of freedom which are taken into account using an intramolecular potential function. The calculated Raman and infrared frequencies are in agreement with the available experimental data and the analysis of the polarization vectors allows us to characterize the experimental spectra. A soft-mode branch with a minimum at q=0.80, b^* appears which could explain the transition to the incommensurate phase where the modulation can be described with a wave vector of 0.78, b^* at 90, K. The soft-mode eigenvector is compared with the modulation displacements of the incommensurate phase and the influence of the anticrossing behaviour which shows the soft branch near its minimum is discussed.