Study of the phase transition of benzil by simultaneous Raman-scattering and domain-structure investigations

Abstract

Backward Raman measurements of external modes between 300 and 5 K are reported. For the first time the existence of an underdamped soft optical mode above and below the transition temperature (83.5 K) is evidenced. The soft mode of the high-temperature phase, of $E$ symmetry, possibly induces the transition. Simultaneous studies of the domain structure in the low-temperature phase lead to the identification of $W$ and $W^{\prime }$ walls. The orientation of the $W^{\prime }$ walls gives the ratio $\frac{a}{c}$ of the independent components of the spontaneous strain tensor of the low-temperature phase which according to symmetry considerations is ferroelectric as well as ferroelastic. Complementary measurements of 90° Raman scattering and dielectric-constant temperature variations are also reported.