Laser Brillouin and Raman Scattering Investigation of the Melting Transition in the Molecular Crystal Salol C(13)H(10)O(3) (phenyl Salicylic Acid)

Light scattering studies of the melting transition in phenyl salicylic acid commonly known as salol have been performed. Using a 5145(ANGSTROM) argon ion laser, both Brillouin and Raman scattering experiments on salol were performed very close to the melting point and down to room temperature. In the Brillouin scattering measurements, the velocity and attenuation of the acoustic modes have been determined to within 0.01(DEGREES)C of the melting point (T(,m)(TURN)41.0(DEGREES)C). Single-pass and triple -pass interferometers were used which yielded a working finesse of 50 and 70 respectively. Within 0.01(DEGREES)C of T(,m) both a decrease of Brillouin shift and an increase in linewidth were observed in all three Brillouin modes of salol. The observed sound velocity decreased to 80% of its room temperature value near T(,m). The most prominent result was the observation of a rapid increase of Brillouin linewidth near T(,m). This result was interpreted by a simple dislocation model, in which the concentration of thermally activated dislocations was assumed proportional to the observed hypersonic attenuation. By taking the temperature dependent dislocation density as (delta)D(T)(TURN)e(-E(,D)(T)/K(,B)T), the temperature dependence of the dislocation formation energy E(,D)(T) was found to be continuously decreasing towards T(,m), indicating the onset of rapid growth of dislocations near T(,m), indicating the onset of rapid growth of dislocations near T(,m) as predicted by the dislocation theory of melting. Through room temperature Brillouin scattering measurements on six different samples, the complete set of elastic and elasto-optic coefficients of salol was determined for the first time. Pockel's coefficients were calibrated with quartz via benzene. The determined refractive indices at (lamda) = 5145(ANGSTROM) were: n(,x) = 1.5473 (+OR-) .0265, n(,y) = 1.812 (+OR-) .0233 and (eta)(,z) = 1.6513 (+OR-) .0145. Raman scattering experiments showed (TURN)5% softening of some salol optical phonons near T(,m). The observed change in relative intensities of the Raman modes suggested molecular rotation during melting. The observation of up to eight new modes near T(,m) which were present in neither the room temperature crystal nor in the liquid spectra at T(,m), were attributed to activation by thermally generated defects near T(,m) which further strengthens the dislocation theory interpretation of the melting phenomenon. The lack of a fully structured theory of melting limited the interpretation of the results of this experiment to qualitative and semiquantitative analysis.