Misroscopic observation of phase transitions of ammonium nitrate

Abstract

Phase transitions IV ⇌ II, IV ⇌ V and II ⇌ V of ammonium nitrate were observed through a polarizing microscope by using thin single crystals of various orientations. In the transition II ⇀ IV, a single crystal is split into two kinds of crystals, in which the direction of the c-axes is common but the a- and b-axes are interchanged. A composite pattern appears which is characteristic of the orientation of the initial crystal. The crystal composites have boundaries of {110} planes, which may be twinning planes. On the reserve transition, the composites are unified again. In both transformations, the new phase grows in a specified direction in the mother crystal in conformity with a minimum strain postulate. The IV ⇌ V transitions are very similar to the IV ⇌ II transitions except for the appearance on occasions of a faint chiaroscuro pattern of phase V. This pattern is probably due to the formation of microtwins. The II ⇌ V transitions occur as a second order transition in minute crystals within a temperature range from about 44°C to 41 °C. The transformation has been detected only by the same chiaroscuro pattern of phase V in some crystals as in the case of the IV ⇀ V transition. The observed transition phenomena are discussed on the basis of the crystal structures of the polymorphs of ammonium nitrate.