Ultrasonic Investigation of the Lambda Transition in NH4Br

The ultrasonic-attenuation coefficient $\alpha$ has been measured in the disordered phase of N${\mathrm{H}}_4$Br at 10, 30, and 50 MHz as a function of temperature for the [100] longitudinal wave and the [110] transverse wave associated with the $c^{\prime }$ shear. By combining these data with available ultrasonic- and hypersonic-velocity data it is possible to determine the dynamical behavior near the order-disorder transition if one assumes a single-relaxation model. The critical relaxation time for the longitudinal wave is well represented by $\frac{1}{\tau }\left(\mathrm{in}\mathrm{units}\mathrm{of} {10}^8 invsec\right)=1.8+1780\mathrm{}{\epsilon }^{1.0}$, where $\epsilon$ is the reduced temperature $\frac{(T-T_{\lambda })}{T_{\lambda }}$. The transverse relaxation time is quite similar. The critical attenuations themselves can be approximately represented by a power law of the form ${\alpha }_c=S{\omega }^2{\epsilon }^{-l}$, where $l\simeq 1.35$. It was found possible to obtain an essentially single domain N${\mathrm{H}}_4$Br crystal in the tetragonal ordered phase by cooling the crystal slowly with a small temperature gradient along one of its axes. Longitudinal and shear velocities were measured in the ordered phase as a function of temperature from 120 °K to $T_{\lambda }\simeq 234.5$ K. Critical attenuation measured at 10 MHz for the longitudinal wave was well described by a power law with the exponent $l=0.75\mathrm{}$.