Brillouin and raman scattering studies of the melting transition in salol

Brillouin and Raman scattering studies of salol from room temperature to within 5 mK of the melting transition at T_m = 40.97°C are reported. Changes in the Brillouin shifts and linewidths were accurately determined by nonlinear least-squares fitting and deconvolution. A marked increase in the deconvoluted Brillouin linewidth (~400%) and a gradual softening (~20%) of the transverse acoustic modes were observed very close to T_m. The increase of the Brillouin linewidths was analyzed by a simple dislocation model assuming the hypersonic attenuation to be proportional to the concentration of thermally generated defects near T_m. The defect formation energy E_D(T) was computed from the temperature-dependent linewidth data, and was found to decrease significantly (~60%) near T_m, suggesting a cooperative effect producing a catastrophic growth of defects which brings about melting by destroying the long range order of the crystal. The conclusion that melting is mediated by a sudden growth of defect concentration near T_m was further strengthened by Raman scattering experiments in which 13 new Raman modes appeared close to T_m. These new modes are believed to be defect activated through breaking of the local symmetry of the crystal. A slight softening of the Raman modes (~5%) was observed close to the melting point.