DYNAMICAL STUDY BY RAMAN-SCATTERING OF THE FERROELECTRIC PHASE-TRANSITION OF THE DISUBSTITUTED DIACETYLENE 1,6-BIS(2,4-DINITROPHENOXY)-2,4-HEXADIYNE (DNP)

Dynamical study by Raman scattering of single monomer crystals of 1,6- bis-(2,4-dinitrophenoxy)-2,4-hexadiyne (DNP) has been performed in the low- and high-temperature phase (T(c) almost-equal-to 46 K). A soft m ode characterized by a weak and constant damping is clearly observed i n the low-temperature phase; this structural instability presents a di splacive character. The critical exponent has been determined from the variation of other low-frequency modes which are coupled with the ord er parameter. Interesting additional information on the molecular conf ormation is given by the study of high-frequency modes. These results are in good agreement with dielectric and calorimetric measurements an d they indicate either a tricritical or a weakly first order transitio n.