Heat Capacities and Thermodynamic Properties of Ammonium Nitrate Crystal: Phase Transitions between Stable and Metastable Phases

The heat capacities of ammonium nitrate were measured between 15 and 410°K for the stable and metastable phases by use of adiabatic- and conduction-type calorimeters. The phase transformations between stable phases; i.e., V–IV, IV–III, III–II and II–I, and the metastable one between IV and II, were all found to be of first-order type. The direct transition between V and II, which is only realized for the sample treated with a minute amount of surface active agent, was ascertained to be typical higher-order type. Heat and entropy of transition of five kinds of transitions were determined. These results conclude that the heat capacity data by Stephenson et al. at low temperature region are much more reliable than the recent values by Eichenauer et al. The possibility of the existence of the seventh phase below 156°K has been proposed.