High resolution Raman study of the soft mode in the incommensurate bis(4-chlorophenyl)sulfone

The soft optical mode of bis(4-chlorophenyl)sulfone is analysed up to the incommensurate transition temperature Ti with both a tandem of Fabry-Perot interferometers and a triple monochromator. High resolution spectra (∼4 GHz ) of the soft mode reveal a mean field behaviour up to Ti - 4 K, but between Ti and this temperature the frequency of the soft mode saturates whereas its linewidth increases and its intensity decreases. The occurrence of second order scattering with a very slow decrease of its intensity above Ti precludes the explanation of the observed soft mode blocking by a divergence of the second order scattering. Different spectral profiles of the soft mode have been recorded in the (bb) and (γγ) polarizations, γ being the principal dielectric axis parallel to the molecular long axis. Within single mode analysis, excess intensity is found between the soft mode and the longitudinal acoustic mode in the (γγ) polarization. This intensity is assigned to a coupling effect between these modes whereas a phason contribution is ruled out for reasons of symmetry and temperature behaviour.