On the phase transition in benzil

The phase transition in crystalline benzil [(C6H5CO)2] at 84 K is investigated through Brillouin scattering. The major experimental findings are two transverse acoustic phonon modes exhibiting softening near the phase transition, a longitudinal acoustic mode that is temperature sensitive, and the ratio of the Rayleigh peak intensity to the soft mode intensity behaves anomalously. These results are discussed using a theory developed for an elastic phase transition in sym-triazine (C3N3H3). It is found that bilinear-coupling terms involving strains and the order parameter can explain transverse mode behavior but not that of the longitudinal acoustic mode.