Elastic Constants of Ammonium Chloride near the Lambda Point

Abstract

The adiabatic elastic constants of single‐crystal ammonium chloride have been measured as functions of sound frequency from 5 to 55 Mc/sec, and as functions of temperature from ∼150° to 300°K by an ultrasonic pulse technique. The values of the constants at 300°K are c₁₁ = 3.70, c₄₄ = 0.86, (c₁₁—c₁₂)/2 = 1.41 in units of 10¹¹ dyn cm^—2. Special emphasis was given to the region around 243°K, the critical temperature for the order—disorder transition. The results are discussed in terms of the phenomenological thermodynamic theory of Pippard.