Critical ultrasonic behavior near the normal-incommensurate phase transition inNaNO2

The acoustic velocity and attenuation of the $c_{22}$ longitudinal wave have been measured as a function of frequency (10–74 MHz) and temperature (165–180 °C) in single-crystal sodium nitrite. Critical behavior in the high-temperature disordered (normal) phase near the normal-incom- mensurate phase transition can be well described in terms of a phenomenological dynamic-scaling theory. The static sound velocity exhibits a temperature dependence like that expected for the heat capacity. The velocity dispersion and the attenuation are well described with a critical relaxation time τ=(1.55×${10}^{\mathrm{-}11}$)$t^{\mathrm{-}1}$, where t=(T-$T_c$)/$T_c$. The velocity and attenuation exhibit the expected scaling behavior, and their ratio satisfies the Kramers-Kronig relation.