Brillouin-scattering investigation of the ferroelastic transition of benzil

The acoustic properties of benzil are investigated by means of Brillouin scattering in both high- and low-temperature phases. The elastic constants have been measured in the trigonal phase as a function of temperature from 300 to 84 K; it appears that the constant $c_{44}$ is the most sensitive to the phase transition. The transverse acoustic mode of lowest energy which propagates along the $x$ axis is a soft acoustic mode related to the same order parameter as the Brillouin-zone-center $E$ soft optical mode. In the low-temperature monoclinic phase, measurements have been performed in the $z$ direction for the longitudinal and transverse lines: the variation with temperature of the frequency and linewidth of these modes has been observed to be continuous at the phase transition.