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

Abstract

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 NaNO₂ 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.