Raman study of external frequencies of thiourea with temperature

Below 193K, the Raman spectra of thiourea displays a new line whose frequency omega is given by the relation omega ²=A(T₀-T) with A=9.2-or+1 cm^-2 K^-1 and T₀=199.5+or-2K. This line results from the softening of a translational B_3u mode of thiourea at normal temperature. The mode of frequency omega (N mode) becomes polar below 169K and its contribution to the static dielectric constant suggests that the transition at 169K can be attributed to an order-disorder rotational process very weakly coupled with the N mode.