Order—Disorder Phenomena. III. Effect of Temperature and Pressure on the Elastic Constants of Ammonium Chloride

• Published in: The Journal of chemical physics 1966-02, Vol.44 (3), p.1130-1139
• Main Authors: Garland, Carl W., Renard, Rémi
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.1726798

The adiabatic elastic constants of single-crystal ammonium chloride have been measured at 20 Mc/sec as functions of temperature and pressure in the region of the lambda transition. At atmospheric pressure, data were obtained over the range of temperature from 150° to 320°K. At five temperatures, evenly spaced between 250° and 310°K, measurements were made as the pressure was varied from 0 to 12 kbar. The values at 300°K and atmospheric pressure are: c11 = 3.815, c44 = 0.8878, C′ = 1.4698, in units of 1011 dyn cm−2. A hysteresis of the critical temperature, equal to ∼0.9°K at atmospheric pressure, was observed; this hysteresis disappears rapidly as the hydrostatic pressure is increased. The results are in good agreement with those predicted for a compressible Ising model. In particular, it is concluded that the order—disorder transition in ammonium chloride is not of a simple lambda type. It is proposed that NH4Cl is unstable in the immediate vicinity of its critical point and undergoes a first-order transition.