No abstract

We report on the study of the pretransitional dynamics of the structural phase transition in anthracene-TCNB by Raman scattering and inelastic neutron scattering at atmospheric pressure. This mixed-stack charge-transfer molecular crystal presents an antiferrodistorsive structural phase transition ͑SPT͒ corresponding to the reorientation of the anthracene molecules in their molecular plane. The dynamical regime of this SPT appears to be mainly displacive. However, our measurements clearly show different behavior of soft mode parameters in both phases. This particularity could be interpreted in terms of an intermediate regime between displacive and order-disorder regimes as was proposed in a coupled double-well potential model ͓S. Aubry, J. Chem. Phys. 64, 3392 ͑1976͔͒. An intense central peak is also found in the neutron spectra just above the transition temperature. It presents a critical behavior in Q space clearly different from the soft-mode one.