Central peak near the incommensurate displacive phase transition of BCPS

We report on a high resolution neutron study of the dynamical response of bis (4-chlorophenyl) sulfon near its displacive structural phase transition. On the basis of previous estimations of the soft mode characteristics , we show that for this molecular compound the narrow central peak arising at the transition can be partially energy resolved. The central line can be described as composed of, at least, two contributions with different energy scales: a weak relaxation in the 10 GHz frequency range and a main component, elastic within the neutron energy window. These results are analyzed in the light of existing microscopic models