ACOUSTIC DISPERSION OF (NH3C2H5)(2)MNCL4 NEAR THE STRUCTURAL PHASE-TRANSITION AT 226 K

The anomaly in the longitudinal elastic constant C-33 of a (NH3C2H5)(2 )MnCl4 layered compound around the structural phase transition at T-2 = 226 K was investigated by the ultrasonic method (f = 15 MHz) and by low frequency dynamic mechanical analysis (f = 0.5- 12 Hz). The presen t results show a very pronounced dispersion of the longitudinal acoust ic modes which are explained in the frame of a phenomenological model including both the effect of temperature fluctuations (isothermal-adia batic crossover) and order-parameter relaxation. The temperature depen dence of the order-parameter relaxation time has been determined from ultrasonic measurements as tau(eta) = tau(0)/(T-2 - T), with tau(0) = 4.5 x 10(-10) s K. This value explains the absence of the negative dip anomaly in previous Brillouin scattering experiments. The thermal rel axation time is of the order of tau(th) approximate to 0.1 s, and is s trongly dependent on sample thickness.