Influence of uniaxial stress on ultrasonic wave propagation near the ferroelastic phase transition in LiCsSO4 crystal

Changes in velocity and attenuation of the longitudinal ultrasonic waves induced by the uniaxial stresses sigma X, sigma Y and sigma Z in LiCsSO4 crystal in the vicinity of ferroelastic phase transition, are studied. Far from the phase transition, the stress derivatives of the relative velocity changes, being measures of nonlinear elasticity, are calculated. The stresses applied change the stepwise nature of the velocity changes at TC as a result of the nonlinear elasticity. The uniaxial stresses applied change the phase transition temperature too. The stress sigma X acting on the wave propagating in [0 1 0] direction drastically decreases the stepwise change in the wave velocity up to its disappearance at 2 MPa and in the temperature range between TC and TC-6 K it leads to increase in the wave velocity and decrease in its attenuation. This range was interpreted as that of co-existence of ferroelastic and incommensurate ordering.