Raman phonon spectroscopic study of phase transition in trans-stilbene crystal

Raman phonon spectroscopy in the 10–170 cm−1 range has been used to study phase transition in trans-stilbene crystal. At 77 K, eight distinct phonon modes and an internal mode are observed whereas at 300 K the spectrum shows only four phonon bands and an internal mode. The temperature dependence of spectral pattern and some phonon band width suggest that the phase transition in this crystal is quasi-continuous over a small temperature range 215–225 K. The absence of a soft mode and a strong increase of the phonon band width, when approaching the Tc from below, suggest that the phase transition is of order-disorder type. The well established disordered structure of trans-stilbene crystal at room temperature attains an ordered structure at low temperature at least through one, possibly two order–disorder transitions.