Vibrational Spectroscopic and Dielectric Studies of some Ferroic Crystals

Abstract

The polarized Raman spectra of LCS are studied in the room temperature orthorhombic and the low temperature monoclinic phases. The observed Raman lines are assigned in the C(aa)B setting. The polarized spectra agree well with the group theoretical predictions based on the factor group symmetries D?h and D? of the two phases.

The study of the external modes in different polarization settings across the phase transition did not show the presence of any soft optical phonon mode. Based on this, it can be said that the phase transition in LCS is likely to be a true proper ferroelastic one, in which case the order parameter of the transition is the spontaneous strain that is developed in the ferroelastic phase.

The internal mode A_g bands in the symmetric and asymmetric bending mode regions did not show any noticeable changes across the phase transition. Our Raman spectroscopic study of both the external and internal modes across the phase transition reveals that a continuous phase transition associated with gradual structural changes is taking place in LCS.

The observed anomaly in the total integrated area of the totally symmetric V? mode in (aa) and (ac) polarizations across the transition is attributed to the increase of fluctuations in the polarizability of the scattering species. From the differential scanning calorimetric trace of LCS, the transition enthalpy is found to be 0.213 kJ/mol. In the room temperature infrared spectrum, the observed absorption bands are assigned to the B?g and B?u symmetry species.