Abstract:
<jats:p>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.</jats:p>
Description:
<jats:p>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.</jats:p>