Heat Capacity of Ammonium Chloride between 8 and 300 K

Heat capacity of NH4Cl was measured between 8 and 300 K with an adiabatic calorimeter. The low-temperature limit of the Debye temperature is 352 K and its high-temperature limit in the harmonic approximation is 285 K. The λ transition point Tλ is 242.502±0.004 K with the heat of transition 1168 J mol−1 and the entropy of transition 5.00 JK−1 mol−1. It is very close to the first-order transition. From the analysis of the heat capacity between 242 and 300 K, an extra contribution to the heat capacity that amounts to 11.5 JK−1 mol−1 at 300 K is extracted and explained by a simple model which assumes an additional type of order-disorder in the arrangment of NH4 ions above Tλ. The energy difference between the parallel and the antiparallel pairs of NH4 ions is estimated to be 1.46 kj mol−1 from the low-temperature tail of the heat capacity anomaly near Tλ.