Optical and thermal studies of transitions between phases II, III and IV of ammonium nitrate

Transitions between phases II, III and IV of ammonium nitrate are studied in some detail using microscopy, dilatometry, electrical conductivity and differential scanning calorimetry (d.s.c.). Clear evidence of a two-step IV–III transformation is presented and the competition between phase transitions IV–II and IV–III–II is explained in terms of a kinetic versus thermodynamic control. A similar explanation accounts for the inhomogeneity of typical specimens of pure ammonium nitrate, as demonstrated by many examples of ‘mixed transitions’.