THE III-IV PHASE-TRANSITION IN AMMONIUM-NITRATE - MECHANISMS

The polymorphic phase transition III-IV in ammonium nitrate is well kn own to be catalysed by water. Owing to this predominant role of water, it has been suggested in the literature that this transition proceeds via dissolution and crystallization occurring at a disordered phase b oundary or even in a ''bulk'' layer of solution. A systematic study on the similarity between projections of specific (hkl)III and (hkl)IV f aces predicts various orientation relationships between both phases. S everal of these (hkl) faces and orientation relationships are confirme d by microscopic observations of the III --> IV and IV --> III transit ions, by texture goniometry results, as well as by microscopic observa tions reported in literature. The combined experimental and theoretica l results substaintiate the existence of a solid-solid transition mech anism especially for single crystal. The transition is supposed to be continuously catalysed from the surface of the crystals by solvent mol ecules. The presence of a solution layer at the transition front would be unlikely in view of the proposed solid-solid mechanism which impli es a greater restriction than just a set of parallel faces, namely the requirement of matching of faces.