BRILLOUIN AND ULTRASONIC STUDIES OF PHASE-TRANSITIONS IN CS2CDBR4 .1.EXPERIMENTAL

The phase transitions in Cs2CdBr4 have been studied by means of Brillo uin scattering and ultrasonic measurements. The sequence Pnma <-> 252 K incommensurate <-> 235 L P2(1)/n11 gives rise to significant anomali es of both real and imaginary parts of the elastic constants. In the c ase of C44, a strong asymmetric broadening of the Brillouin lines was observed in the IC phase and several degrees above the Pnma <-> IC tra nsition. The differences between Brillouin and ultrasonic results conc erning C22 and C44 suggest a strong frequency dependence of the proces s governing the above-mentioned sequence of phase transitions. A low-t emperature transition at 156 K has been studied; it presents a conside rable softening of shear stiffness constants. Selection rules for Bril louin scattering are greatly affected at the transitions to monoclinic and triclinic phases, allowing to measure the shear stiffness constan ts with a good accuracy. A detailed discussion of the phenomena linked to the monoclinicity is provided. In addition, the ultrasonic experim ents have confirmed the existence of a previously reported phase trans ition at 208 K.