Raman and Brillouin-Rayleigh scattering studies of the displacive phase transition in malononitrile at 294.7 K

A comprehensive temperature dependent inelastic light scattering study of a malononitrile single crystal using Raman and Rayleigh-Brillouin spectroscopic techniques has been carried out, with a special emphasis on the phase transition at 294.7 K, observed by Zussman and Alexander. Optical soft modes associated with the phase transition have not been observed, despite the fact that internal vibrations involving the CH₂ scissoring motion and the symmetric C-N stretching vibrations are affected by the phase transition. The librational frequencies have been assigned for the first time, using the present Raman result and the low temperature heat capacity data. Among the three librational modes, the libration about the c axis, corresponding to ν_L(B_g, c), shows anomalies in both the frequency and the linewidth data. The temperature dependent Rayleigh-Brillouin scattering spectra have displayed the softening of the lowest frequency transverse acoustic mode at 294.7 K. A combination of the data on the librational mode, ν_L(B_g, c), with the Brillouin-Rayleigh results suggests the importance of coupling of the transverse acoustic mode to the ν_L(B_g, c) libration in malononitrile single crystal. Evidence for the coupling is also manifest in the line shape of the depolarized Rayleigh scattering spectra in various crystal orientations. The linewidth and intensity of the central Rayleigh peak as well as the linewidth of the longitudinal acoustic phonon were also found to be affected by the phase transition.