Ultrasonic investigations of incommensurate Rb2Zncl4

Elastic properties of Rb₂Zncl₄ were investigated by measuring the velocity and attenuation of both, transversal and longitudinal ultrasonic waves propagating along the crystallographic axes. At the lock-in phase transition temperature only v₄₄ shows a remarkable anomaly. This anomaly has been found to be asymmetric with respect to the interchange of oscillation and propagation direction of the transverse ultrasonic wave. The anomaly of v₄₄ at T_c is proportional to the contents of commensurate regions in the IC structure. From the temperature dependence of ultrasonic velocities at the phase transition from paraelectric to IC phase the critical behaviour of the order parameter should correspond rather to the (n = 2)-Heisenberg-model than to the Ising model.