Ultrasonic studies of incommensurate phase transitions

The temperature dependencies of the ultrasonic velocities and attenuation coefficients are studied in the vicinity of the incommensurate phase transitions of BCCD and AHSe crystals. The largest contribution to the longitudinal ultrasonic anomalies is due to the coupling to the amplitude mode, whose relaxation time τ diverges as the transition temperature T i is approached with the temperature dependence given by:τ= τ 0 T/(T - T i ). The amplitudon relaxation time for AHSe crystal (τ 0 = 5.1 10 -2 s) was found to be considerably longer than for BCCD (τ 0 = 1.3.10 -13 s). From the measurements of shear ultrasonic wave velocity in incommensurate phase below Ti = 164 K the critical exponent that describes the order parameter temperature behaviour has been calculated for BCCD. The value β = 0.345 is consistent with three dimensional X - Y model. The substitution of Cl by Br in BCC 1-x Br x D crystals with x < 0.24 manifests itself in smearing of the elastic anomalies and snifting T i to lower temperatures with increase of Br concentration.