On the phase transition in benzil

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Abstract
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.