Order—Disorder Phenomena. IV. Ultrasonic Attenuation near the Lambda Point in Ammonium Chloride

The attenuation coefficient α of longitudinal acoustic waves propagating along the [100] direction in ammonium chloride has been reinvestigated as a function of frequency from 10 to 60 Mc/sec and of temperature from 215° to 300°K. Special emphasis was given to 10-Mc/sec data at temperatures very close to the order—disorder transition, and hysteresis was observed. These attenuation data are consistent with the behavior expected from a compressible Ising model which predicts a mechanical instability in the immediate vicinity of the transition temperature.