Ultrasonic Study of NaNO2. II. Dynamics of Amplitude Mode in Incommensurate Phase

At various ultrasonic frequencies the attenuation coefficient and the sound velocity together with the dielectric constant were simultaneously measured near the incommensurate-and-antiferroelectric phase transition temperature ( T N ). The difference between T N and the temperature where the maximum attenuation coefficient appeared was obtained as a function of frequency. According to the theories of Landau-Khalatnikov and of Bruce-Cowley, the results could be explained in terms of the critical slowing-down of the amplitude mode below T N . In the case of the plane wave limit as in NaNO 2 the phase mode is not coupled with the acoustic mode contrary to the amplitude mode and thus no information about the diffusion-type phase mode could be drawn.