The Variation of the Adiabatic and Isothermal Elastic Moduli and Coefficient of Thermal Expansion with Temperature through theλ-Point Transition in Ammonium Chloride

Measurements of the temperature variation of the adiabatic and isothermal Young's and rigidity moduli and of the coefficient of thermal expansion of pressed specimens of ammonium chloride in the neighborhood of the $\lambda$-point transition at 242.8°K are reported. It is found that a large difference exists between the adiabatic and isothermal compressibilities below the critical temperature, and hence a large difference between the specific heat at constant pressure and the specific heat at constant volume. The data, in conjunction with the known temperature variation of the specific heat at constant pressure, permit the evaluation of the temperature variation of the specific heat at constant volume, and from this it is concluded that the transition is not to be associated, as previously suggested by Pauling, with the occurrence of almost free rotation of the ammonium radicals in the crystal lattice.