STUDY OF THE PHASE-TRANSITION OF BENZIL BY SIMULTANEOUS RAMAN-SCATTERING AND DOMAIN-STRUCTURE INVESTIGATIONS

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′ walls. The orientation of the W′ walls gives the ratio ac 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. © 1979 The American Physical Society.